UPPER TUOLUMNE RIVER ECOSYSTEM PROJECT Preliminary Sediment Source and Sediment Transport Capacity Evaluation: O Shaughnessy Dam to Poopenaut Valley

Transcription

1 UPPER TUOLUMNE RIVER ECOSYSTEM PROJECT Preliminary Sediment Source and Sediment Transport Capacity Evaluation: O Shaughnessy Dam to Poopenaut Valley Technical Memorandum McBain & Trush, Inc. and RMC Environmental Prepared for: The San Francisco Public Utilities Commission, Water Enterprise, Natural Resources and Lands Management Division and Hetch Hetchy Water and Power Division June 13, 2008

2 About the Upper Tuolumne River Ecosystem Project In June 2006, the San Francisco Public Utilities Commission (SFPUC) adopted the Water Enterprise Environmental Stewardship Policy and directed the Water Enterprise to integrate this policy into the planning and operation of the SFPUC water system infrastructure, including Hetch Hetchy Project dams and diversions. The policy establishes a management directive to protect and rehabilitate ecosystems affected by water system operations, within the context of meeting water supply, power generation, water quality, and existing minimum instream flow requirements. The policy further directs the nature of SFPUC instream flow releases such that they mimic, to the extent feasible, the variation of the seasonal hydrology (e.g., magnitude, timing, duration, and frequency) of their corresponding watersheds in order to sustain the aquatic and riparian ecosystems upon which native fish and wildlife species depend. Subsequent to adoption of the Environmental Stewardship Policy, the SFPUC initiated the Upper Tuolumne River Ecosystem Project with the goal of conducting a set of long-term, collaborative, science-based investigations designed to (1) characterize historical and current river ecosystem conditions, (2) assess their relationship to Hetch Hetchy Project operations, and (3) provide recommendations for improving ecosystem conditions on a long-term, adaptively managed basis. Primary partners include the SFPUC, Yosemite National Park, Stanislaus National Forest, and the U.S. Fish and Wildlife Service. The study area includes reaches of the Upper Tuolumne River mainstem and major tributaries regulated by the Hetch Hetchy Project, from O Shaughnessy Dam to Don Pedro Reservoir, Cherry Creek downstream of Cherry Valley Dam, and Eleanor Creek downstream of Eleanor Dam. This document prepared by: McBain & Trush, Inc., Arcata, California and RMC Environmental Prepared for: The San Francisco Public Utilities Commission, Water Enterprise, Natural Resources and Lands Management Division and Hetch Hetchy Water and Power Division Acknowledgements We would like to thank Dr. Greg Stock, geologist for Yosemite National Park, for his assistance in providing background information, reference materials, and valuable field discussions, and Laura Clor for her field assistance. We also thank National Park Service Archives staff for their help in obtaining aerial photographs of the study reach. Preferred citation McBain & Trush, Inc. and RMC Environmental Upper Tuolumne River Ecosystem Studies: Preliminary Sediment Source and Sediment Transport Capacity Evaluation O Shaughnessy Dam to Poopenaut Valley. Technical Memorandum. Prepared for the San Francisco Public Utilities Commission. June 13, 2008.

3 Table of Contents 1 Introduction and Background Objectives Geomorphology Overview Sediment Budget Input (Sediment Supply) Output (Sediment Transport) Changes in Storage Study Sites Methods Sediment Source Evaluation Sediment Transport Capacity Evaluation Results and Discussion Sediment Sources (Input) Sediment Transport Capacity (Output) Considering changes in sediment storage Linking Results to Objectives Literature Cited...15

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5 1 Introduction and Background Prior to the completion of O Shaughnessy Dam in 1923, sediment was delivered to the Hetch Hetchy Reach 1 of the Tuolumne River by a combination of hillslope and fluvial processes. Rainfall and runoff eroded and delivered sediment from watershed hillslopes to tributary channels, which transported their sediment loads to the Tuolumne River. Climatic variations caused this erosion and transport to fluctuate over moderate timescales (hundreds to thousands of years) but in general, sediment eroded from the watershed was transported out by the Tuolumne River such that watershed erosion and sediment delivery rates to the Tuolumne River were generally in balance with the fluvial transport and sediment export from the basin. This condition can be called a dynamic quasi-equilibrium. The equilibrium was disrupted following completion of O Shaughnessy Dam. It was previously hypothesized that the pre-dam sediment supply was naturally low due to a selective trapping of coarse sediment in the low-gradient Hetch Hetchy Valley (McBain & Trush and RMC 2007). The degree of this trapping with respect to the total sediment supply is unknown. In April 2007, McBain & Trush (M&T) performed a reconnaissance-level geomorphic investigation between O Shaughnessy Dam and Poopenaut Valley, hereafter referred to as the study reach (Figure 1). Within the study reach, the mainstem Tuolumne River has bedrock-controlled channel morphology. As the Tuolumne River enters Poopenaut Valley, the channel transitions from bedrock-controlled to an alluvial morphology (McBain & Trush and RMC 2007). Observations made during this reconnaissance investigation showed that sediment storage conditions (i.e., sediment deposits and aggradation in the channel) were substantial. This finding contradicted typical conceptual models of sediment storage downstream of dams often in the relevant literature. For example, geomorphic impacts of reduced sediment supply often include channel incision, bed coarsening, and reduced coarse sediment storage in the downstream channel. Such conditions were observed for a short distance downstream of O Shaughnessy Dam, but sediment storage increased with distance downstream. Sediment storage in Poopenaut Valley appeared disproportionately high (primarily sand and fine gravel). These observations of unexpectedly high sediment storage were summarized in M&T and RMC (2007), and more detailed study was recommended to better evaluate the sediment story and explain the observed sand and fine gravel aggradation. This report follows the M&T and RMC (2007) recommendations by performing a more detailed investigation to better understand the historical and contemporary sediment story in the study reach. A better understanding of sediment supply and transport will help to evaluate contemporary sediment storage in the project reach, which can then serve as a basis to estimate potential changes under future flow management scenarios. Because sediment deposits typically provide the physical template for aquatic and riparian habitat in riverine ecosystems, understanding flow regime-driven sediment supply, transport, and storage changes will be critical in developing environmental flow recommendations for O Shaughnessy Dam. 1 The Hetch Hetchy Reach is defined as a reach of the mainstem Tuolumne River between O Shaughnessy Dam and the Cherry Creek confluence. See McBain & Trush and RMC 2007 for maps and descriptions of designated study reaches within the Upper Tuolumne River between O Shaughnessy Dam and Don Pedro Reservoir. 1

6 2 Objectives The objective of this study is to evaluate hypotheses presented in M&T and RMC (2007) within an approximately four mile reach of the Upper Tuolumne River between the O Shaughnessy Dam and Poopenaut Valley. The two hypotheses presented in M&T and RMC (2007) specific to the study reach have been refined as follows: (1) Sediment storage (coarse and fine sediment) on the Tuolumne River upstream of Poopenaut Valley has decreased slightly due to the presence of O Shaughnessy Dam. Although the sediment transport capacity has been reduced, the reduction in sediment supply has likely been greater than the reduction in sediment transport capacity, such that storage has decreased slightly, and; (2) Fine sediment storage available for mobilization on the Tuolumne River within Poopenaut Valley has likely decreased due to the presence of O Shaughnessy Dam, yet changes in storage and grain size may still be modest because large amounts of sand and fine gravel persist as alluvial features in the lower portion of the study reach, and are most pronounced in Poopenaut Valley. Questions remain regarding how these deposits relate to contemporary sediment supply and sediment transport, whether or not there is there a local sand source, and whether or not existing sand deposits are aggrading, scouring, or are in balance with the sediment budget. Based on the above hypotheses, the following three objectives were developed to evaluate sediment storage in the study reach and assess potential changes in sediment storage from future flow management operations: (1) Evaluate sediment supply to determine if significant sediment sources are present that can help explain why observed fine sediment storage in the channel remains high. (2) Evaluate pre- and post-dam sediment transport capacity to help explain why observed sediment storage remains high. (3) Develop a method using sediment transport capacity modeling for evaluating potential changes in sediment storage under different flow management scenarios. A combination of qualitative information (observation-based) and quantitative analyses (modeling-based) are used to evaluate these objectives. 3 Geomorphology Overview Before describing the processes governing sediment supply and transport, a more detailed evaluation of the study reach geomorphology was conducted to build on the observations and interpretations presented in M&T and RMC (2007). This evaluation provides necessary information for evaluating sediment supply, storage, and transport. M&T and RMC (2007) described the overall channel gradient for the study reach as moderately steep, having a slope of approximately 1.4 percent; however, the actual channel profile is comprised of a series of stepped higher- and lower-gradient subreaches, ranging from very steep and turbulent chutes to broad, deep pools, which is a characteristic glaciated valley channel morphology (Flint, 1971). Unlike an alluvial channel where the bed and banks are made of sediments (alluvium) and the channel is formed and maintained by the fluvial processes that erode and deposit these sediments, the channel in the study reach is controlled structurally by bedrock. The bedrock governs the channel s planform location 2

7 and its overall gradient, and also creates local channel constrictions and expansions that create the higher-gradient and lower-gradient reaches (respectively). Higher-gradient subreaches are confined, have relatively low sediment storage, and are dominated by coarser particle sizes including gravel, cobble, and boulder (Figure 2). In contrast, lower- gradient subreaches begin at valley expansions and end where the channel encounters a new bedrock constriction (Figure 3). Some lower-gradient subreaches contain very deep pools at their upstream ends, visually estimated to be greater than 30 feet deep. Bedrock constrictions at the downstream end of the lower-gradient subreaches commonly create hydraulic grade controls, facilitating a backwater effect during high flows, and promoting fine sediment deposition along banks and on terraces (small gravel, sand, and finer sediments). Compared to the higher-gradient subreaches, the lower-gradient subreaches are dominated by finer sediments, including fine gravel and sand, with some silt and possibly clay in overbank floodplain deposits. Progressing downstream from O Shaughnessy Dam, the alluvial subreaches exhibit an overall fining trend in particle size distributions (the percentage of coarser sediments decreases and the percentage of finer sediments increases, such that few coarse sediments are visible on the bed or banks downstream of a very steep and incised reach located approximately 1,500 ft upstream of Poopenaut Valley). Like Hetch Hetchy Valley but at a much smaller scale, the alluvial subreaches appear to trap the coarser sediments, progressively filtering them from the sediment load and resulting in a significantly finer particle size distribution at Poopenaut Valley. This is shown conceptually in Figure 4. Poopenaut Valley itself represents the largest of the lower- gradient subreaches in the study reach. Previous reconnaissance described the lower-gradient subreaches as having a sand veneer (M&T and RMC 2007), inferring a relatively thin (i.e., inches) layer of fine sediments overlies a coarser bed material. More recent observations suggest the observed finer sediments are substantially thicker (i.e., feet) and have buried the coarser sediments. In general, the lower-gradient subreaches upstream are comprised of a mixture of sand, gravel, and cobble, while the lower-gradient subreaches in the vicinity of Poopenaut Valley are comprised primarily of sand and fine gravel. These observations are consistent with the results of in-channel sediment sampling (see Section 6.2). Downstream of Poopenaut Valley, the Tuolumne River enters a steep bedrock canyon where coarse sediment supply is assumed to increase (assumed primarily from rockfall but also from occasional tributaries). This reach was inaccessible and observations of channel sediments were not made. 4 Sediment Budget A sediment budget is an estimate of the rates of production, transport, and discharge of sediment from its point of origin to its eventual exit from the drainage basin. Sediment budgets are often quantitative, derived by applying extensive sediment source inventories, calculating transport rates and delivery volumes, and examining the interrelationships between transport processes and hillslope form to determine sediment yield from a basin. This study does not compute a sediment budget for the study reach; rather, it evaluates the primary components of a sediment budget to address study objectives. Although a sediment budget is not being computed, an understanding of the concept and components, and how 3

8 the components are expressed in terms of contemporary channel geomorphology, provides the context for evaluating potential changes in sediment storage. A sediment budget can be defined as: I ± O = S where: I = sediment input (supply), O = sediment output (transport), and S = change in sediment storage (channel aggradation or incision). Examples of each variable as they pertain to the project reach include: coarse sediment supply from hillslopes and tributaries (I), fluvial transport by the Tuolumne River (O), and alluvial features such as gravel bars and floodplain deposits (S). Each of these variables and their associated processes in the study reach are described in more detail below. 4.1 Input (Sediment Supply) Because O Shaughnessy Dam traps all sediment delivered from the watershed above the dam, the sediment supply downstream of the dam has been reduced. The volume trapped by O Shaughnessy Dam is unknown, no volumetric estimates of pre-dam or post-dam sediment yield were found (e.g., from a sediment yield analysis), and thus no pre-dam versus post-dam comparison has been made. The trapping efficiency of Hetch Hetchy Reservoir is likely to be nearly 100 percent, although a small amount of fine suspended sediments (silts, clays) may pass the dam primarily during times when inflow to the reservoir is very high. Because the amount of sediment passing the dam is likely a very small proportion of the total sediment load, it is assumed sediment supply (Input) equals zero immediately downstream of the dam and gradually increases in the downstream direction as additional sediment sources (hillslopes, tributaries, rockfalls) contribute to the sediment budget. 4.2 Output (Sediment Transport) The output component of the sediment budget equation is determined by the rate of sediment transport out of the reach. The sediment transport rate can be estimated using a variety of coarse sediment transport equations (e.g., Parker et al. 1982, Parker 1990), or empirically using bedload samplers (e.g., Bunte and Abt 2001), or by using volumetric methods (e.g., McBain and Trush 1997). Of all methods, the volumetric method is most accurate. This method requires physical sediment sampling to measure bedload and suspended sediment, and requires large equipment such as a boat and a boat-and-crane mounted sediment sampler. This equipment could not be feasibly brought to the sampling sites, nor was such a detailed sampling program part of this investigation. Bedload sampling can provide reasonable estimates of coarse sediment transport rates under favorable hydraulic conditions, but sampling can be costly. Sediment transport equations predict the rate of sediment transport based on hydraulic conditions (slope, channel geometry, channel roughness) and bed surface particle size distribution. These equations typically assume that the stream is not limited by sediment supply, and that the source of the sediment originates from the bed itself (Parker et al. 1982). In mountain streams however, the steepness of the channel results in the ability to carry a much greater volume of sediment than what the actual sediment supply can support, thus mountainous streams are usually supply limited. 4

9 Because sediment transport equations compute the channel s theoretical transport capacity based upon the assumption that supply is unlimited, the resulting sediment transport predictions are usually much larger than the actual sediment transport rates. However, a predicted sediment transport capacity can be used as a transport index, which provides a useful tool for evaluating theoretical sediment transport rates for given streamflows and comparing differences between flows (assuming the transport capacity differences between flows are analogous to the actual sediment transport differences). 4.3 Changes in Storage Changes in storage result from the differences between sediment supply and sediment transport. For a sediment budget to be balanced, the volume of sediment input by the watershed must equal that transported and removed by the stream (I = O and therefore S = 0) (Figure 5). Typically, dams cause negative channel storage immediately downstream of the dam (O > I), often expressed by bed coarsening and channel incision. Field observations suggested bed coarsening and incision (inferred from portions of exposed bedrock channel) extended for a short distance downstream of the dam, but these effects become offset further downstream as observed sediment storage gradually increased. Although the channel is cut into bedrock, sediment supplied to the channel is routed downstream and stored as alluvial features such as bars, floodplains, and lee deposits. These features are common throughout the study reach. 5 Study Sites Based on the simplified model of two subreach types (higher- and lower-gradient) and on the geomorphic characteristics found in each subreach type, two sites were selected to evaluate sediment storage and sediment transport capacity: the USGS gaging station study site and the Poopenaut Valley study site (Figures 6 and 7). These two sites were selected by inventorying the entire study reach for long, straight reaches. The relatively simple hydraulics of long, straight reaches improves sediment transport modeling results. Although each of the two selected study site reasonably fit these conditions, they are not ideal for assuming simplified hydraulics. However, they are the best available and provide a good representation of typical channel conditions. The USGS gaging station study site was selected to represent sediment transport conditions in the higher-gradient subreach type. The bed particle size distribution ranges from boulder to gravel, with little sand. The representative high flow water surface slope surveyed through this reach is (0.91%). The Poopenaut Valley study site was selected to represent sediment transport conditions for the lower-gradient alluvial channel morphology subreach type. The bed particle size distribution ranges from fine sand to fine gravel, and the representative high flow water surface slope surveyed through this reach is (0.06%). 6 Methods To better understand why observed sediment storage in the study reach appears higher than expected, a sediment source evaluation was conducted to qualitatively evaluate sediment supply (Input), and sediment transport capacity was modeled to provide quantitative estimates (indices) of sediment transport (Output). These results were then compared to help explain theoretical storage changes and compare with field observations. The methods used to perform these evaluations are described below. 5

10 6.1 Sediment Source Evaluation Sediment storage within the study reach was evaluated using a combination of office- and field-based methods. Office methods included reviewing topographic maps, geologic maps, aerial photographs and ground photographs. U.S. Geological Survey topographic maps were reviewed to identify tributary channels that could be responsible for supplying sediment to the reach. Geologic maps (Dodge and Calk 1987; Huber et al., 1989) were reviewed to understand the bedrock and surficial geology in the study area and to assess what geologic units could be responsible for supplying coarse and fine sediment to the study reach. Historical and recent ground photographs were reviewed to compare channel changes preand post-dam and to review contemporary sediment storage conditions. Low-altitude aerial photography was taken of the study reach in August 2007 but was not available for review prior to the field evaluation. The next most recent color aerial photographs of the project reach were taken in 1997, which were used to attempt identification of sediment sources in the project reach watershed area. Color infrared scans of these photographs were obtained from the National Park Service (1:15,860 scale, or 1 inch equals approximately 1,300 feet) and reviewed using a mirror stereoscope to identify visible sediment sources such as rockfalls or debris slides. This review was somewhat limited by the image scan quality. Although the photographs were scanned at the highest available resolution, the magnification by the mirror stereoscope caused the images to pixelate, making magnified viewing difficult. It was difficult to identify with certainty any potential sediment source smaller than approximately 100 feet long or wide. In addition, because the photographs are 10 years old, it is possible that more recent sources could have occurred since the photos were taken. However, the stereoscopic projection provided a helpful watershed overview and allowed for the identification of tributary valleys that could likely represent sediment sources. In August 2007, a field reconnaissance was made to follow-up on the office review described above. The field reconnaissance was aimed at evaluating sediment supply and storage in the study reach, comparing field observations with office observations and evaluating the initial hypotheses contained in M&T and RMC (2007). M&T and National Park Service (NPS) geologists visited the entire reach from O Shaughnessy Dam to Poopenaut Valley, reviewing maps, taking photographs, and discussing the geologic history of Hetch Hetchy Valley and contemporary geomorphic processes. 6.2 Sediment Transport Capacity Evaluation Sediment transport capacity was calculated to evaluate flow-induced changes to sediment transport in the study reach. Because the overall study reach was grouped into one of two distinct subreach types (the higher-gradient subreach type represented by the USGS gaging station study site and the lower-gradient subreach type represented by the Poopenaut Valley study site), sediment transport capacity was evaluated separately for each subreach type. Sediment transport capacity was estimated using the BAGS (Bedload Assessment in Gravelbedded Streams) software package developed by Wilcock et al. (2007) which calculated sediment transport rates using the sediment transport equation of Wilcock and Crowe (2003). The Wilcock and Crowe equation was chosen primarily because of the model s ability to include particle sizes finer than 2 mm (bed surface particle size distributions at the Poopenaut Valley site contained approximately 30 percent sand finer than 2 mm). 6

11 Sediment transport equations are based on a ratio of shear forces acting on the channel bed (flow), and the resistance provided by the channel bed and banks (roughness). This ratio, expressed numerically, is particle size and streamflow-specific. The BAGS program uses site-specific channel geometry, substrate, and hydrologic information to compute a sediment transport capacity rating curve that accounts for the entire range of particles on the bed and the changing hydraulics over the range of selected streamflows at a given cross section. All input data were obtained from data collected at the two study sites in April At each study site, a cross section was defined and surveyed to document the channel geometry. Cross sections were surveyed using standard field protocols (e.g., Harrelson et al. 1994) (Figures 8 and 9). Longitudinal profiles of water surface and high water marks from the Spring 2006 high flow release were surveyed up- and downstream of the cross sections to estimate representative reach-averaged water surface slopes and energy gradients. A modified Wolman surface pebble count (Leopold 1970) was conducted at the USGS gaging station site, and bulk samples were collected using a modified McNeil-type sampler (Bunte and Abt 2001) at the Poopenaut Valley site; particle size distributions for both sites are shown in Figure 10. Hydrologic data used to represent the range of flows for which transport capacity was estimated were calculated from USGS published daily average flowduration analyses representing three distinct flow regimes: pre-dam ( ), postdam without Canyon Tunnel diversion ( ), and post-dam with Canyon Tunnel diversion ( ). Daily average flow duration curves for each flow regime are shown on Figure 11. Input data were run through the BAGS program using the Wilcock and Crowe (2003) transport equation to generate coarse sediment transport capacity rating curves, which estimated the sediment transport rate (tons/day) as a function of streamflow (cubic feet per second) for any given flow at each study site. Using these curves, the annual sediment transport capacity was calculated for each water year of record (tons/year), and the results were grouped by flow regime (pre-dam, post-dam no Canyon Tunnel diversion, post-dam with Canyon Tunnel diversion) to determine differences between each period. 7 Results and Discussion The following sections present results from the sediment source evaluation (Section 7.1) and results from the sediment transport capacity evaluation using the BAGS modeling (Section 7.2), followed by discussion of how the sediment transport capacity results are used to evaluate changes in sediment storage (Section 7.3). The discussion also explains how this evaluation can be used to help explain why observed sediment storage is high, as well as provide a means for further evaluating potential changes in sediment storage under different flow management scenarios. 7.1 Sediment Sources (Input) With exception of a hillslope stabilization project that was completed in 2006 (which represents a small localized source of rounded sediments ranging in size from boulder to sand [Faulkner 2007]), sediment supplied to the channel immediately downstream of O Shaughnessy Dam is limited to coarse angular rock primarily from rockfall. These rocks are stored in the channel along with remnant (pre-dam) rounded cobbles and boulders infrequently mobilized by contemporary flows. This type of coarse and angular bed is typical in rivers immediately downstream of dams. Progressing downstream, sediment supply includes additional mechanisms (e.g., debris flows), but sediments remain coarse 7

12 (gravel, cobble, boulder) and the angularity becomes offset as rounded rocks begin to dominate the bed material. Just upstream of the USGS gaging station study site and approximately 4,000 feet downstream of O Shaughnessy Dam, the bed particle size shifts to a finer distribution with the first appearance of sand stored in the channel, marking a significant shift in bed particle size (Figure 5). From this location and continuing downstream, two trends were observed: (1) fine sediment storage gradually increases (e.g., increasing number of bars, lee deposits, overbank deposits), and (2) mean bed particle size gradually decreases. Combined, these observations show that as sediment supply (Input) continues with distance downstream through the study reach, sand and fine gravel storage increases downstream to Poopenaut Valley while coarser sediment storage decreases (shown conceptually in Figures 12, 13, and 14). Based on these observations, sand and fine gravel supply and storage is proportionally greater than coarse sediment supply and storage in the study reach. Although several likely sediment source areas were identified from the office review of maps and aerial photographs, no single location was found in the field that contributed large quantities of sediment to the channel (no major sand or gravel sources). Because of this, sediment supply in the project reach is described on a more general basis as opposed to site-specific features. Two primary generalized sediment sources were identified in the project reach: hillslopes (including tributaries), and the channel itself (from the bed and banks). Published geologic mapping (Dodge and Calk 1987; Huber et al., 1989) shows a suite of Pleistocene-age (including Tioga, Tahoe, and pre-tahoe) lateral moraines and unconsolidated glacial deposits perched along the crest of the southeast valley wall (Figure 15). Further discussions during a field investigation with NPS geologist Dr. Greg Stock also revealed that much of the moraine sediments (i.e., glacial till) mantle a significant portion of the southern valley slope, well beyond the limits of the moraines shown by the geologic mapping (Figure 15). Much of this till remains in-situ, and some has been transported further downslope by hillslope processes (e.g., debris flows). Glacial till typically contains a range of particle shapes and sizes, from boulders to sands, silts, and clays. Although the moraines were not visited, they likely represent the source for the vast majority of contemporary sediment supplied to the study reach. Sediment supply directly from the hillslopes occurs by mass wasting and surface erosion, including rockfall, debris slides, soil creep, and from tributary channels. Given the location of the moraines on the southeast valley wall, and given that several unnamed tributaries have their headwaters originating in the moraines and flow downslope through till-mantled hillslopes (Figure 15), it is likely that these tributaries represent an important sediment source to the study reach The second primary sediment source in the study reach is the channel itself. Sediments are stored as alluvial deposits, including bars and lee deposits in the channel, cobble and gravel lag deposits on flat bedrock shelves adjacent to the channel, and finer overbank deposits on floodplains and alluvial terraces, most commonly occurring at valley expansions. These deposits, primarily composed of sand and fine gravel, are easily erodible and very mobile. Many floodplains and terraces at valley expansions contain exclusively sand and fine gravel. These deposits are essentially cohesionless (loose and very soft underfoot), and thus require very little hydraulic force (shear stress) to mobilize them. Evidence of the spring 2006 high flow was observed throughout the study reach on these fine sediment surfaces, often expressed as debris lines that formed a small sediment berm, leaving an obvious high 8

13 water mark (Figure 16). The sediment berm shows how easily the fine sediments can be entrained and their high recruitment potential. Although it is clear the spring 2006 high flow mobilized (and presumably scoured) fine sediments on many overbank surfaces, it is less clear whether sediment redeposition is replenishing what was scoured. Some surfaces suggest little change (redeposition following scour) whereas other surfaces show greater change (scour with little redeposition). Regardless of whether a given location is aggrading, degrading, or in balance, overbank sediments represent fine sediment source areas available for transport when their surfaces are inundated. 7.2 Sediment Transport Capacity (Output) Sediment transport capacity rating curves are shown for each study site in Figure 17 (USGS gaging station study site) and Figure 18 (Poopenaut Valley study site). Assuming that: (1) channel geometry, particle size, and water surface slope have remained constant at the study reaches pre- and post-dam, and (2) changes in sediment supply to the study reach remains unchanged (i.e., continues at pre-dam rates), any changes in predicted sediment transport capacity occur from changes in streamflow. Using the sediment transport capacity rating curves, the average annual sediment transport capacity was calculated at each study site for each flow regime (pre-dam, post-dam no Canyon Tunnel diversion, post-dam with Canyon Tunnel diversion) to determine differences between each period. Results are shown in Table 1 below. Flow regulation has changed streamflows such as flow timing, flood magnitude, and flood frequency. For example, a flood frequency analysis of the USGS Tuolumne River near Hetch Hetchy, CA gaging station (gage no ) has shown shifts in flood magnitude and frequency following the completion of O Shaughnessy Dam in 1923, and then again after the completion of Canyon Tunnel in 1967 (Table 2). These changes had moderate impacts to common floods (1.5-year recurrence) and smaller impacts to larger, less frequent floods (M&T and RMC 2007). These hydrologic differences also directly affected the timing and frequency of sediment-transporting flows, and may have increased or decreased sediment storage in the study reach. In comparing differences between periods, the pre-dam record is noticeably shorter than the post dam record. Previous analyses by M&T and RMC (2006) estimated a longer-term pre-dam flow record by scaling flow data from the Merced River at Pohono Bridge near Yosemite gaging station. This estimated pre-dam record was found to underestimate historical Tuolumne River flows and was not used for this study; instead, the 13 years of gaged records was used. As a result, the predicted pre-dam sediment transport capacity results shown in Table 1 must be interpreted cautiously due to the relatively short streamflow record (13 years) compared with the much longer post-dam record. The results in Table 1 represent a first-step analysis intended to illustrate overall changes. 9

14 Table 1. Summary of Changes in Predicted Coarse Sediment Transport Capacity. Flow Regime Period Pre-dam Post-dam, no Canyon Tunnel diversion Post-dam, with Canyon Tunnel diversion USGS gaging station study site average annual sediment transport capacity (tons/yr) Percent change (%) Poopenaut Valley study site average annual sediment transport capacity (tons/yr) Percent change (%) 553, , ,000-54% 1 100,000-28% 1 173,000-32% 2-69% 1 1 Percent change from pre-dam flow regime. 2 Percent change from post-dam, no Canyon Tunnel diversion flow regime. 46,000-54% 2-67% 1 Table 2. Summary of Annual Peak Flood Magnitude Estimates from the USGS Gaging Station Tuolumne River near Hetch Hetchy, CA (USGS ) for the Pre-dam Period, Post-dam Period With No Canyon Tunnel Diversion, and the Post-dam Period With Canyon Tunnel Diversion (from M&T and RMC 2007). Recurrence Interval (years) Pre-dam ( ) b Estimated Flood Magnitude (cfs) a Post-dam, no Canyon Tunnel diversion ( ) Percent Change from Pre-dam Post-dam, with Canyon Tunnel diversion ( ) Percent Change from No Diversion 1.5 8,300 4,730-43% 1,780-62% ,500 6,900-19% 5,100-26% 5 10,150 9,450-7% 8,000-16% 10 11,200 10,600-5% 9,900-7% 25 N/A c 12,100 N/A 14, % Footnotes: 1. All estimates made from interpolations of raw data due to poor curve fitting of Log-Pearson III distribution. 2. Period of record extended from to by adjusting annual peak data for Tuolumne River at Hetch Hetchy near Sequoia ( ) by 1.19 based on peak flow-to-peak flow ratio between the two gages for those two overlapping years. Estimates considered poor due to short period of record and supplemental data. 3. Record too short to make reasonable estimate. 10

15 7.3 Considering changes in sediment storage The study reach from O Shaughnessy Dam to Poopenaut Valley can be characterized by: (1) reduced streamflows (sediment transport capacity), (2) a reduced sediment supply (trapping by O Shaughnessy Dam), (3) a longitudinal decrease in particle size, and (4) a longitudinal increase in fine sediment storage volume. From a sediment budget perspective, both Input (I) and Output (O) have been reduced. Input reductions are solely attributed to trapping by O Shaughnessy Dam, assuming supply from the watershed downstream of the dam remains equal to pre-dam rates. Assuming that Hetch Hetchy Valley trapped all coarse sediments (similar to that observed in Poopenaut Valley but to a much larger extent) and the volume passing through Hetch Hetchy Valley to the study reach was low in proportion to other sources (e.g., hillslopes), overall coarse sediment supply reductions due to O Shaughnessy Dam are assumed small but fine sediment reductions are more significant. Similarly, sediment transport capacity modeling results show Output reductions. Flow regulation has caused substantial decreases in sediment transport capacity (Table 1). As discussed previously, field observations show I < O for the first 4,000 feet downstream of O Shaughnessy Dam, but then shift closer to I = O as transport capacity decreases and observed sediment storage gradually increases down to Poopenaut Valley. Qualitatively, potential changes in sediment storage can be described from field observations. The observed longitudinal particle size gradient and sediment storage in the study reach, shown conceptually in Figures 12, 13, and 14, suggests the greatest geomorphic impacts to the study reach extend to a point approximately 4,000 feet downstream of O Shaughnessy Dam. In this uppermost portion of the study reach, sediment supply is low and bed coarsening has likely occurred by flows removing the finer transportable fraction of the bed material (i.e., fine gravel and sand have been winnowed out with no replenishment). In an alluvial river, the channel would likely incise as streamflows recruit additional sediment, but the bedrock channel prevents this from occurring, and as a result the channel remains steep and coarse. Further downstream (approximately 4,000 feet downstream of O Shaughnessy Dam), the effects of the dam appear to be offset by increasing fine sediment supply and decreasing sediment transport capacity (slope); therefore, we see increasing sediment storage in the channel. Continuing downstream to Poopenaut Valley, the combination of continued fine sediment supply and low transport capacity, coupled with a downstream fining in mean bed particle size, results in the channel having a much finer particle size distribution and has allowed large storage features (e.g., large sandy bars) to remain. Compared with the upper portion of the study reach (near the USGS gaging station study site), the lower portion of the study reach has proportionally greater sediment storage with the size and frequency of the storage features increasing with distance downstream, ending with large sandy bars in the vicinity of Poopenaut Valley, and Poopenaut Valley itself representing the largest storage area in the study reach. In addition to field observations of sediment supply and storage, a limited comparison of historical ground photographs at Poopenaut Valley (presented in M&T and RMC 2007) suggests little change between pre-dam and contemporary sediment storage. With exception of vegetation changes (e.g., conifer encroachment), very little differences can be seen in the overall volume of sediment stored in the valley. This may suggest contemporary flows and sediment supply are nearly balanced at Poopenaut Valley (i.e., reduced transport capacity is proportional to reduced supply); however, without additional ground or aerial photographs showing other sections of the study reach, it is unknown how these other sections have responded post-dam. For example, are the large sandy bars upstream of 11

16 Poopenaut Valley post-dam, post- Canyon Tunnel diversion features? If not, have they grown in size or changed in composition? No photographs other than those described in M&T and RMC (2007) and the 1997 aerial photographs were reviewed. If other historical aerial or ground-based photographs are available, additional analysis could be done to document the abundance and size of the bars, which could help better estimate the magnitude of shift in the sediment budget (e.g., whether the large sandy bars in the Poopenaut Valley vicinity formed in response to reduced transport capacity, or whether they are pre-dam features that have see little storage volume change). 7.4 Linking Results to Objectives The following sections revisit each study objective. Sections and review sediment supply and sediment transport in terms of both coarse and fine sediment, and Section describes a method for using the sediment transport rating curves to evaluate potential transport and storage changes under different flow management scenarios Objective #1: Sediment storage as a function of sediment supply The first objective of this study is to evaluate sediment supply in the study reach to determine if significant sediment sources are present that can help explain why observed sediment storage in the channel remains high. No significant sediment sources are assumed to have initiated in the project reach since the completion of O Shaughnessy Dam in 1923, and thus downstream of the dam, post-dam sediment supply to the channel is assumed the same as pre-dam supply minus the volume trapped by O Shaughnessy Dam. To help explain observations of coarse and fine sediment storage downstream of O Shaughnessy Dam, we offer the following: Coarse sediment supply: Coarse sediment supply reductions due to O Shaughnessy Dam are assumed small, because Hetch Hetchy Valley likely trapped virtually all coarse sediments. As such, the pre-dam supply of course sediments passing through Hetch Hetchy Valley to the study reach was naturally low. Progressing downstream through the study reach, predam coarse sediment supply has always been very low, being limited to hillslope supply (rockfall, debris flows), minor erosion of coarse sediment stored on bedrock terraces, and course sediment contributions from small tributaries. Reduction of high flows may have slightly reduced coarse sediment supply by reducing magnitude and frequency of coarse sediment recruitment from the terraces, but would have had no effect on hillslope or tributary sources. The combination of low coarse sediment supply and longitudinal coarse sediment attrition results in essentially no coarse sediment storage in the bed surface in Poopenaut Valley greater than 3 inches in diameter. If the pre-dam Hetch Hetchy Valley had similar slope as Poopenaut Valley, it would have also screened out these larger sizes of coarse sediment. Fine sediment supply: In contrast to our hypothesis for coarse sediment, the pre-dam Hetch Hetchy Valley likely had a much smaller effect on trapping fine sediment. Because Hetch Hetchy Valley was likely less effective at trapping fine sediments than course sediments, fine sediment supply reductions to the project reach due to O Shaughnessy Dam are likely more substantial. Our field observations found single point sources that could be responsible for supplying the majority of fine sediment currently stored in Poopenaut Valley (as evidenced by storage in the channel). For example, downstream of O Shaughnessy Dam, we looked for tributaries, landslides, and eroding hillsides/terraces that may provide fine sediment supply downstream of the dam; however, our field observations did not 12

17 identify any large-scale fine sediment sources except for the remaining fine sediment storage within the pre-dam floodway (bars and terraces). Contemporary fine sediment storage in Poopenaut Valley is high and likely remains high for one of two reasons: (1) Fine sediment supply equals or slightly exceeds fine sediment transport (I > O), or (2) Fine sediment transport slightly exceeds fine sediment supply (O > I), but transport rates are sufficiently low such that sediments are slowly being removed from predam storage deposits upstream of Poopenaut Valley, and the effects have not fully propagated through the reach (and thus reductions in sediment storage are difficult to observe geomorphically). Based on the information available at this time, we suspect there is a small deficit caused by flows slowly mining the remaining fine sediment storage in the valley, and thus hypothesize the latter is most likely. Recommendations for additional sediment supply and storage evaluation: To evaluate the scale of coarse and fine sediment trapped by Hetch Hetchy Reservoir, we recommend the following: (1) Investigating whether bathymetry surveys (done in 2001) can be used to accurately estimate the volume of fine sediment trapped in Hetch Hetchy Reservoir (representing the volume lost to the reach downstream of O Shaughnessy Dam). This volume (supply) can then be compared to the computed sediment transport capacity rates to obtain ratios of transport capacity to supply for the three flow regimes. (2) Investigating whether the pre-dam topographic surveys through Hetch Hetchy Valley are precise enough to compare with the recently-surveyed Poopenaut Valley slopes. In addition, perform field reconnaissance and slope measurements of Yosemite Valley to evaluate how it traps coarse and fine sediment, and how the reach downstream of the valley looks (via coarse and fine sediment storage) compared to the reach downstream of O Shaughnessy Dam Objective #2: Sediment storage as a function of sediment transport capacity The second study objective is to evaluate pre- and post-dam sediment transport capacity to help explain why observed fine sediment storage is high. The sediment transport model used for the sediment transport capacity analysis (Wilcock and Crowe 2003) accounted for the full range of bed particle sizes and therefore separate transport capacity estimates for coarse and fine sediment populations were not possible. Reductions in total sediment transport capacity correspond with flow regulation, first following the completion of O Shaughnessy Dam (the post-dam, no Canyon Tunnel diversion period) and then again following the completion of Canyon Tunnel in 1967 (the 1968-present post-dam with Canyon Tunnel diversion period). For sediment storage to increase, these reductions in sediment transport capacity must be greater than any reductions in sediment supply such that supply exceeds transport (I > O). As described in the Objective #1 discussion above, changes in the post-dam era coarse sediment supply can be considered negligible but reductions in overall sediment transport capacity are substantial, calculated as 32 percent and 54 percent of pre-dam conditions (at 13

18 the USGS gaging station and Poopenaut Valley study sites, respectively). These reductions as they pertain to coarse and fine sediment are described below: Coarse sediment transport: While sediment transport has decreased, the low coarse sediment supply is still well below transport capacity, creating very little storage changes. The computed reductions in sediment transport capacity, combined with field observations of coarse sediment storage, suggest transport exceeds supply (O > I) (Figure 19). Fine sediment transport: The sediment balance shift for fine sediment is likely more substantial than for coarse sediment due to the greater impact of O Shaughnessy Dam on fine sediment supply. The computed reductions in sediment transport capacity combined with our field observations of fine sediment storage suggest the study reach has remained near equilibrium (Figure 19). Using the conceptual graph shown in Figure 19, any reductions in sediment supply (I) and/or sediment transport (O) cause a departure from the balanced sediment budget. Although field observations, combined with sediment transport capacity model results, suggest reductions in both sediment supply (coarse and fine) and sediment transport, a more accurate portrayal of the shift requires improved estimates of supply changes to the project reach (as recommended in the previous section). Recommendations for additional sediment storage study: The transport capacity calculations suggest that the reduction in sediment transport, despite the likely substantial reductions in fine sediment supply, has allowed fine sediment storage to remain high in Poopenaut Valley. Because we do not know whether the sediment budget is in equilibrium, or whether there is a small deficit, we recommend further evaluation of this hypothesis by: (1) Using the upcoming high resolution aerial photographs and photogrammetry-based cross sections to estimate remaining volumes of fine sediment stored in the bars, and divide that volume by the post-dam sediment transport capacity rates to estimate the number of years of remaining sediment storage. This assumes (a) future valley wall and tributary fine sediment sources are negligible, and (b) fine sediment supply is limited and is not replenishing / maintaining the bars. In other words, contemporary high flows are assumed to be eroding the bars, but how long will the bars persist if they are not being replenished (10 years, 100 years, 1,000 years)? (2) Obtaining 1930 s aerial photographs and comparing planform area of fine sediment storage features (bars) with the 2007 aerial photographs to evaluate whether fine sediment storage (inferred from planform area of bars) has been reduced over time, or has remained consistent over time Objective #3: Method using sediment transport capacity modeling to evaluate future change The third study objective is to develop a method using sediment transport capacity modeling results that can be applied to evaluate potential changes in sediment storage under different flow management scenarios. With the coarse sediment transport capacity rating curves computed, sediment transport capacity can be computed for future flow management scenarios and compared with results presented in this report. Using the sediment transport rating curves, additional analyses can be done to improve understanding of sediment storage, such as evaluating sediment transport capacity over different water years, which will help predict the role of different water year classes on 14

19 sediment transport, and help evaluate spatial and temporal changes in the study reach sediment balance (I versus O). 8 Literature Cited Bunte, K. and S.R. Abt Sampling surface and subsurface particle-size distributions in wadable gravel- and cobble-bed streams for analyses in sediment transport, hydraulics, and streambed monitoring, Fort Collins, CO, U.S. Dept. of Agriculture, Forest Service, Rocky Mountain Research Station Vol. 428 Dodge, F.C.W. and L.C. Calk Geologic map of the Lake Eleanor Quadrangle, Central Sierra Nevada, California. Department of the Interior, U.S. Geological Survey. Map GQ- 1639, scale = 1:62,500. Faulkner, L Tuolumne River Channel Clearing. California Builder and Engineer, Vol 115, no. 8, pgs 6-9. Flint, R.F Glacial and Quaternary Geology. John Wiley & Sons, Inc., New York. 892 p. Harrelson, C.C., C. L. Rawlins, and J. P. Potyondy Stream channel reference sites: an illustrated guide to field technique, General Technical Report RM-245, USDA Forest Service Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO. 61 p. Huber, N. K., P. C. Bateman, and C. Wahrhaftig. Geologic Map of Yosemite National Park and Vicinity, California. USGS Miscellaneous Investigations Series Map I-1874, scale = 1:125,000. Leopold, L. B An improved method for size distribution of stream bed gravel, Water Resources Research, 6(5), McBain and Trush, Inc., and RMC Water and Environment Upper Tuolumne River: Description of River Ecosystems and Recommended Monitoring Actions. Final Report, April McBain and Trush, Inc., and RMC Water and Environment Upper Tuolumne River: Available Data Sources, Field Work Plan, and Initial Hydrology Analysis. Technical Memorandum prepared for San Francisco Public Utilities Commission, San Francisco, CA. McBain and Trush, Trinity River Channel Maintenance Flow Study Final Report, Prepared for the Hoopa Valley Tribe, Hoopa, CA. Parker, G., P. C. Klingeman, and D.G. McLean, Bedload and size distribution in paved gravel-bed streams, Journal of the Hydraulics Division, Proceedings of the American Society of Civil Engineers, Vol. 108, No. HY4, Parker, G Surface-based bedload transport relation for gravel rivers. Journal of Hydraulic Research, Vol. 28, No. 4,

20 Wilcock, P.R., J. Pitlick, and Y. Cui Sediment Transport Primer and BAGS User s Manual, Parts I and II. Draft General Technical Report RMRS-GTR-xxx. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain research Station. 121 p. Wilcock. P.R., and J.C Crowe Surface-based transport model for mixed-size sediment. Journal of Hydraulic Engineering, 129 (2), IN, Wilcock, P.R., J. Pitlick, and Y. Cui Sediment Transport Primer and BAGS User s Manual, Parts I and II. Draft General Technical Report RMRS-GTR-xxx. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain research Station. 121 p. 16

21 Figure 1. Portion of the USGS Lake Eleanor 7.5-minute quadrangle map showing the study reach and vicinity. 17

22 . Figure 2. Example of a higher-gradient subreach with bedrock valley confinement, low sediment supply, and low sediment storage. View is facing downstream, location is approximately 3,500 feet downstream of O Shaughnessy Dam. Figure 3. Example of a lower-gradient subreach at a valley expansion with high fine sediment storage. View is facing upstream, location is approximately 12,000 feet downstream of O Shaughnessy Dam. 18

23 Boulder (> 256 mm) Downstream end of Poopenaut Valley 1,000 Continued fine sediment supply and transport Increased fine sediment supply, coarse sediment attrition O'Shaughnessy Dam "Typical" dam effects Cobble ( mm) Gravel (2-64 mm) 100 Sand first appears, offsets particle size distribution 44,000 46,000 48,000 50,000 52,000 54,000 56,000 58,000 60,000 62,000 64,000 66,000 Distance (ft) upstream of Early Intake Mean bed particle size (mm) Sand (< 2mm) Gravel and cobble disappear, further offsetting particle size distribution Figure 4. Conceptual diagram portraying observed longitudinal changes in particle size, O Shaughnessy Dam to Poopenaut Valley

24 O > I (transport reach) I > O (depositional reach) O I (transport reach) I > O (depositional reach) 3,600 3,550 3,500 3,450 3,400 3,350 3,300 3,250 3,200 3,150 3,100 Steep canyon reach below Poopenaut Valley, assume coarse sediment supply and transport increases. Low sediment supply, low coarse sediment transport but moderate fine sediment transport, high sediment storage (slow post-glacial aggradation). POOPENAUT VALLEY Low sediment supply, high coarse and fine sediment transport capacity, small (but transient) sediment storage. HETCH HETCHY VALLEY Low-gradient, low confinement Hetch Hetchy Valley. Moderate sediment supply, low coarse sediment transport but moderate fine sediment transport, high sediment storage (slow post-glacial aggradation). Elevation (ft) 3,050 Channel profile 3,000 38,000 40,000 42,000 44,000 46,000 48,000 50,000 52,000 54,000 56,000 58,000 60,000 62,000 64,000 66,000 Distance (ft) upstream of Early Intake Figure 5. Conceptual graph showing hypothesized Tuolumne River study reach sediment dynamics prior to O Shaughnessy Dam. The sediment budget is defined as: I ± O = S where: I = sediment input (supply), O = sediment output (transport), and S = change in sediment storage (channel aggradation or incision). For a sediment budget to be balanced, the volume of sediment input by the watershed must equal that transported and removed by the stream (I = O and therefore S = zero). In addition to the overall study reach, the longitudinal channel profile identifies geomorphically distinct subreaches. Channel aggradation (I > O), degradation (O > I), and equilibrium (I = O) conditions are based on the assumed ratio of sediment input to output. The sediment budget for the overall study reach is assumed balanced (I O), but lower-gradient reaches are likely aggradational ( S > zero) over the post-glacial period. 20

25 . Upper Tuolumne River Ecosystem Project Figure 6. Enlarged portion of the USGS Lake Eleanor 7.5-minute quadrangle map showing USGS Gaging Station and Poopeanut Valley study reaches. 21

26 . Upper Tuolumne River Ecosystem Project 3,550 Downstream end of Poopenaut Valley O'Shaughnessy Dam 3,500 3,450 3,400 3,350 Poopenaut Valley study site XS location (surveyed high flow water surface slope = 0.06%) USGS gaging station study site XS location (surveyed high flow water surface slope = 0.91%) Elevation (ft) 3,300 Channel elevation (ft) 3,250 44,000 46,000 48,000 50,000 52,000 54,000 56,000 58,000 60,000 62,000 64,000 66,000 Distance (ft) upstream of Early Intake Figure 7. Longitudinal channel profile (based on USGS topographic map contours) showing stepped higher- and lower-gradient subreaches. 22

27 . Upper Tuolumne River Ecosystem Project 3,470 3,468 Left Bank Looking Downstream Right Bank 3,466 Elevation (ft, NAVD 1988) 3,464 3,462 3,460 3,458 3,456 3,454 3,452 3, Distance from left bank pin (ft) 4/9/07 Ground Surface 4/9/07 Water Surface (Q = 62 cfs at USGS Gage Tuolumne River near Hetch Hetchy ) 6/7/06 HWM Surface (8,170 cfs) Figure 8. Representative cross section used for sediment transport capacity modeling at the USGS Gaging Station site showing topography, April 9, 2007 water surface elevation, and high water mark (HWM) left by the June 7, 2006 flow release (8,170 cfs). 105 Left Bank Looking Downstream Right Bank 100 Elevation (ft, arbitrary datum) Distance from upper left bank pin (ft) 4/10/07 Ground Surface 4/10/07 Water Surface (Q = 62 cfs at USGS Gage Tuolumne River near Hetch Hetchy ) 6/7/06 Peak Water Surface (Q = 8,170 cfs at USGS Gage Tuolumne River near Hetch Hetchy ) Figure 9. Representative cross section used for sediment transport capacity modeling at Poopenaut Valley site showing topography, April 10, 2007 water surface elevation, and a high water mark (HWM) left by the June 7, 2006 flow release (8,170 cfs). 23

28 . Upper Tuolumne River Ecosystem Project 100% 90% USGS gaging station 80% Poopenaut Valley Cumulative percent finer 70% 60% 50% 40% 30% 20% 10% 0% Grain size diameter (mm) Figure 10. Surface particle size distribution at the USGS gaging station and Poopenaut Valley study sites. Samples were collected at the USGS gaging station study site by surface pebble count, and at the Poopenaut Valley study site via bulk sampling. 8,000 7,000 6,000 PRE DAM Daily Average Flow Duration (WY ) POST DAM, NO DIVERSION Daily Average Flow Duration (WY ) POST DAM, WITH DIVERSION Daily Average Flow Duration (WY ) Daily Average Flow (cfs) 5,000 4,000 3,000 2,000 1, % 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Exceedence probability (p) Figure 11. Daily average flow duration curves for pre-dam, post-dam no Canyon Tunnel diversion, and post-dam with Canyon Tunnel diversion flow regimes, Tuolumne River near Hetch Hetchy, CA gaging station (USGS gaging station no ). 24

29 Upper Tuolumne River Ecosystem Project Downstream end of Poopenaut Valley FLOW DIRECTION O'Shaughnessy Dam Sand and fine gravel supply Gravel, cobble, boulder supply High sediment SUPPLY Supply from upstream storage reduced but continues as occasional rockfall Gravel and cobble disappear from sediment load Sand first appears in channel Low sediment SUPPLY 44,000 46,000 48,000 50,000 52,000 54,000 56,000 58,000 60,000 62,000 64,000 66,000 Distance (ft) upstream of Early Intake Figure 12. Conceptualized longitudinal sediment supply gradient, separated into coarser (boulder, cobble, gravel) and finer (sand and fine gravel) size categories. Downstream end of Poopenaut Valley FLOW DIRECTION O'Shaughnessy Dam Sand and fine gravel storage Gravel, cobble, boulder storage High sediment STORAGE Occasional rockfall contributes to local (but largely immobile) storage features Gravel and cobble disappear from sediment load Sand first appears in channel Steep from fine sediment winnowing Low sediment STORAGE 44,000 46,000 48,000 50,000 52,000 54,000 56,000 58,000 60,000 62,000 64,000 66,000 Distance (ft) upstream of Early Intake Figure 13. Conceptualized longitudinal sediment storage gradient, separated into coarser (boulder, cobble, gravel) and finer (sand and fine gravel) size categories. 25

30 . Upper Tuolumne River Ecosystem Project GRAVEL, COBBLE, AND BOULDER, WITH SAND Alternating confined bedrock and unconfined alluvial GRAVEL, COBBLE, AND BOULDER 1 O Shaughnessy Dam Confined bedrock 2 Sand first appears FINE GRAVEL AND SAND Unconfined alluvial Coarse gravel, cobble, and boulder disappear 3 Fine sediment storage increases and mean bed particle size decreases with distance downstream Figure 14. Oblique aerial photograph of the project reach showing observed sediment storage, downstream particle size fining, and conceptual sediment routing. Three subreaches are shown, numbered 1 through 3 on the right side of the image. Reach 1 is a confined bedrock reach with mean bed particle sizes ranging from gravel to boulder. No observed fine sediment sources or in-channel fine sediment storage was observed. Reach 2 is an alternating confined (bedrock) and unconfined (alluvial) reach, characterized by an alternating series of low-gradient reaches with deep pools which progressively filter coarse sediments, such that particle sizes larger than fine gravel are removed from the sediment load. While mean bed particle size becomes increasingly finer with distance downstream, fine sediment storage increases. Reach 3 is an unconfined alluvial reach (Poopenaut Valley) and has a mean bed particle sizes ranging from sand to fine gravel. Coarse sediment supply downstream of Reach 3 is assumed to increase as the channel enters a steep canyon reach. 26

31 . Upper Tuolumne River Ecosystem Project Figure 15. Geologic map of the Lake Eleanor Quadrangle (Dodge and Calk 1987), enlarged to show the project reach. Dashed blue line shows approximate project reach watershed boundary. Numbered arrows show coarse and fine sediment sources: (1) O Shaughnessy Dam eliminates upstream supply, although natural coarse sediment rates from upper watershed are assumed relatively low; (2) Southeast valley sediment supply is largest source to the study reach. Glacial till is labeled (includes dotted moraine crests) and extends further downslope than the limits shown on the map. Sediments included in the till range from boulder to sand (and finer); till is assumed to contribute the majority of fine sediment to the project reach; (3) Northwest valley slope contributes relatively minor sediment, primarily as angular boulder rockfall, and (4) In-channel sediment storage increases with distance downstream of Dam, upstream deposits act as sediment sources for downstream deposits. 27

32 Figure 16. Photograph showing a pine needle and sediment berm debris line (high water mark) located on a right bank terrace, approximately 1,500 feet downstream from the USGS gaging station. The high water mark is shown by the blue arrow, and was formed by the Spring 2006 peak flow event. A previous high flow event formed a scour channel at the back edge of the terrace (shown by the orange arrow). View is facing downstream. 28