Geomorphic Importance of Winter Peak Flows and Annual Snowmelt Hydrographs in a Sierra Nevada Boulder-Bedrock River

Transcription

1 Geomorphic Importance of Winter Peak Flows and Annual Snowmelt Hydrographs in a Sierra Nevada Boulder-Bedrock River Scott McBain and Bill Trush McBain & Trush, Inc.

2 Clavey River and Cherry Creek vicinity map

3 Cross section locations in the Clavey River s Cottonwood Bar subreach

4 Cross section locations in the Clavey River s boulder sub-reach

5 An idealized segment of a boulder-bedrock channel with three levels of nested hydraulic controls and associated depositional features.

6 Stylized aerial photograph of nested hydraulic controls and depositional features within one channel reach of the Clavey River.

7 Clavey River bar

8 Valley wall constriction functioning as a primary hydraulic control

9 Enlargement of previous photograph

10 Generalized flow in bar types

11 Deposition al features movements at various flow recurrences

12 Depositional Feature Aggraded Floodplain Boulder Rib Point Bar Lateral Bar Boulder Cluster Lee Deposit Obstruction Deposit Perched Deposit Pool/Run Tail Deposit Eddy Deposit Definition of Depositional Feature An almost flat or gently sloping surface (away from the thalweg) typically associated with a point bar and created by progressive overbank deposition of silt and sand. Boulders arranged in a transverse line spanning the channel width. A large scale bar usually half a channel meander wavelength long with a relatively short radius of curvature, where the thalweg is located toward the outside bank and coarse bedload is transported across its surface rather than along its thalweg. Cobble and small boulder deposits sheltered from large floods by bedrock protruding from the valley wall or large boulders protruding from the riverbank. Collection of boulders, two or more, each in physical contact with one another. Accumulation of fine/coarse sediment immediately downstream of a roughness element, commonly a single boulder or boulder rib, with the deposit s surface sloping negative, that is, towards the channel bed Accumulation of fine/coarse sediment upstream of a roughness element, commonly with the deposit s surface sloping steeply positive, that is, towards the surface. Accumulation of fine/coarse sediment in local depressions formed by coarser particles or bedrock that is elevated above the thalweg, commonly with the deposit s surface slope appearing flat or reflecting a high flow slope. Fine/coarse sediment deposited at or near a pool or run s downstream control at baseflow stage, with the sediment deposit s surface generally sloping towards the water surface. Fine sediment deposited during late stages of the falling flood limb. Table 1. Depositional features observed on the Clavey River, typical of steep, boulderbedrock Sierra Nevada rivers

13 (B) Aggraded floodplain, adjacent to Cottonwood Bar, in upper right half of photograph. (A) Narrow floodplain as a transition from point bar to aggraded floodplain on Cottonwood Bar in the Clavey River, photographed May 27, 2005

14 A large upstream boulder rib (anchored on right bank bedrock outcrop) and a small downstream boulder rib of modest 4- to 6-ft diameter boulders, that act as upstream hydraulic controls on July 8 and May 27, 2005 in the mainstem Clavey River.

15 Small right bank point bar between widely spaced boulder ribs on XS in the boulder-bedrock sub-reach of the mainstem Clavey River (streamflow right to left).

16 (B) Looking toward the right bank at the same lee deposit (streamflow right to left). (A) Lee deposit along mainstem Clavey River upstream of Cottonwood Creek confluence

17 (B) Looking toward the right bank at same obstruction bar. Note the bar surface slopes upward going downstream (photographed September 01, 2005). (A) Obstruction bar of coarse gravel and small cobble along right bank mainstem Clavey River on XS 35+37

18 Annual recurrences of peak daily average streamflow during the entire water year and during the snowmelt runoff period for the Clavey River

19 Panoramic photograph of the Clavey River mainstem, cross section XS 35+67, looking downstream

20 Sand/gravel likely deposited (lee deposit) by 6.3-yr flood in 1998, photo taken 2005 at Clavey River cross section 35+67

21 Small boulder point bar formed by a 75-yr flood from 1993 to 2000, photos taken downstream of the 1N04 bridge and just upstream of the Clavey River study site boundary

22 General bed mobility limited by two 4.4-yr floods from 2000 to 2005, photos taken in a steep, narrow bedrock reach of the Clavey River looking immediately downstream of the 1N04 bridge

23 Gravel lee deposit and point bar mobility by two 4.4-yr floods from 2000 to 2005, photos taken in the Clavey River boulder sub-reach

24 Small boulder ribs mobilized and re-shaped by a 75-yr flood from 1993 to 2000, photos taken below the Clavey River s confluence with Cottonwood Creek

25 Bed mobility limited by a single 4.4-yr flood from 2003 to 2005, photos taken below the Clavey River s 1N01 bridge

26 Annual maximum flood recurrence (yrs) 1.5 to 3 4 to 6 12 to 17 Annual maximum flood peak magnitude (cfs) 1,300 to 3,700 6,500 to 10,000 18,000 to 23,000 Mobilization actions accomplished Mobilize surfaces (occasionally deeper) of gravel deposits close to the thalweg of pool/run tails and between boulders in boulder ribs not on the thalweg, Deposit/scour/reshape silt and sand eddy deposits, mobilize coarse sand stored in pools, and Likely exchange coarse sand in the matrix between cobbles and small boulders of larger depositional features within the low flow channel and along the flanks (forming one bank of low flow channel) of larger depositional features (e.g., point bars). Deposit sand/gravel onto many smaller depositional features (e.g., lee deposits) in the low flow channel, Deposit coarse sand and small gravel onto point bar/floodplain transition areas, Induce minor gravel scour/re-deposition in pool/run tails and leading edges of large point bars, Maintain side-channels by scouring away sand and gravel, and Selectively mobilize some cobbles in coarser features close to the thalweg. Mobilize/shape/re-shape gravel and cobble depositional features in the low flow channel (e.g., lee and obstruction depositional features), Deposit sand/gravel onto large point bars and floodplain transition areas, Significantly/entirely remove prominent gravel/small cobble pool tail deposits

27 Annual maximum flood recurrence (yrs) Approx to 100 Annual maximum flood peak magnitude (cfs) Approx. 35,000 45,000 to 55,000 Mobilization actions accomplished Scour behind boulder ribs previously buried by bigger floods, Deposit coarse sand onto floodplains and fine sand/silts onto higher depositional features (e.g., aggraded floodplains), Moderately reshape existing side-channels, Pack small boulders/large cobbles against larger boulder ribs, and Create larger woody debris jams capable of influencing local hydraulics of smaller snowmelt floods. Aggrade large point bars and lateral bars with boulders and large cobbles (e.g., as in the upper half of Cottonwood Bar), Form/reshape/eliminate smaller boulder ribs, Scour/construct small point bars, Create/remove side-channels, Deposit sand onto old aggraded floodplains and terraces, and Transport very large boulders short distances that calved from nearby steep bedrock valley walls.