Stop 1: Marmot Dam Stop 1: Marmot Dam

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Donald Oswin Sharp
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1 Stop 1: Marmot Dam Stop 1: Marmot Dam Following the removal of Marmot Dam in 2007, the fate of the reservoir sediments has been monitored through a series of surveys and sediment transport measurements. The post-dam removal monitoring efforts have included: Annual cross-section surveys by David Evans and Associates (DEA) Event-based longitudinal profile surveys conducted by USGS Repeat terrestrial LIDAR surveys conducted by Watershed Sciences USGS made measurements of bedload & suspended load at multiple locations during the winter of Measurement sites included: USGS Cableway near Marmot Dam, Brightwood (above Marmot Dam), Dodge Park & Stark Street Bridge (both located below Marmot Dam) Graham Mathews & Associates (GMA), conducted event-based bedload measurements at Revenue Bridge during the winter of , and at Brightwood

2 Stop 1: Marmot Dam Figure 1 - Overview of Marmot Dam area 13

3 Stop 1a: Marmot Dam - Skybridge Stop 1a: Sky Bridge (Overview, Sediment Transport, Initial Erosion) Figure 2 - Evolution of reservoir immediately upstream from the dam Figure 3 - Evolution in the first year downstream of the dam 14

4 Stop 1a: Marmot Dam - Skybridge Figure 4 - Evolution downstream from the dam Figure 6 Sediment transport in the first 15 year following removal of Marmot Dam

5 Stop 1a: Marmot Dam - Skybridge Figure 7 - Sediment transport measurements immediately following the cofferdam breach 16

6 Stop 1a: Marmot Dam - Skybridge 708 under skybridge Elevation (ft) Station (ft) 696 cableway cross section Elevation (ft) O ct O ct O ct D ec Jan Station (ft) Figure 5 - Cross section evolution below the bridge (top) and at the USGS gauge (bottom) 17

7 Stop 1a: Marmot Dam - Skybridge Figure 6 - Reservoir erosion near the cofferdam, compiled from repeat photography using photogrammetry. A) Longitudinal profile of reservoir near coffer dam 2 hrs after breach; B) Profile of reservoir near coffer dam 72 hours after breach; C) variation in stream width 96 hours after breach. 18

8 Stop 1b: Reservoir (Erosion) Stop 1b: Marmot Dam - Reservoir Figure 7 Photograph of reservoir prior to coffer dam breach; photograph is looking downstream towards the dam. 19

9 Stop 1b: Marmot Dam - Reservoir Figure 11 Repeat longitudinal profile surveys in the vicinity of the dam 20

Stop 2: Beaver Island Stop 2: Beaver Island (Deposition & Sediment Budget) Approximately 800 m downstream of Marmot Dam, the depth of deposited reservoir sediments diminishes.

10 Stop 2: Beaver Island Stop 2: Beaver Island (Deposition & Sediment Budget) Approximately 800 m downstream of Marmot Dam, the depth of deposited reservoir sediments diminishes. A deep pool ~650 m downstream of the dam ( known to local fisherman as the Slaughter Hole ) was filled with gravels in the weeks following the coffer dam breach. Aggradation in the vicinity of the Slaughter Hole has resulted in channel shifting and bar growth. Downstream of the Slaughter Hole, a large floodplain bar is situated on the right bank. This bar was labeled Beaver Island in the 1911 topographic survey (Figure 11) and is still present today..although there is a thick package of newly deposited Marmot Dam gravels near the head of Beaver Island (Stop 2a), most of the deposition in this reach appears as a near parallel increase in the preremoval channel bed. Downstream of Beaver Island (Stop 2b), Marmot Dam gravels have mantled a mid-channel bar. The pre-removal bar consisted of a low surface which was inundated at flows <400 cfs, and contained primarily large clasts. With the addition of sediment from the dam removal, smaller gravel clasts have filled in around the large boulders to form a bar with a larger range of grain sizes. Figure 8 - Changing extent of the bar above Beaver Island 21

11 Stop 2: Beaver Island Figure 9 - Panoramic Photos from above Beaver Island (Stop 2b) - July 2007 (top) and July 2009 (bottom) Figure 14 - Panoramic Photos from below Beaver Island (Stop 2a) - July 2007 (top) and July 2009 (bottom) 22

12 Stop 2: Beaver Island upstream from Beaver Island Elevation (ft Station (ft) downstream from Beaver Island Elevation (ft) Station (ft) Figure 10 - Cross sections from above Beaver Island (top, stop 2b) and below Beaver Island (bottom, stop 2a) 23

13 Stop 2: Beaver Island Figure 11 - Sediment Budget for the Sandy River WY all figures in metric tons 24

14 Stop 3: Oxbow Park Stop 3: Oxbow Park (Sandy River response to non-marmot inputs) Oxbow Park is an excellent location to examine the Sandy River s sediment-rich geologic history, and also provides evidence of recent channel change. Terraces flanking the park s boundaries are testament to major sedimentation events resulting from the river s volcanic and glacial history while more recent evidence of lahars and flood events are preserved in deposits within the active channel. At the upstream end of the park a point bar on the left bank is situated where Buck Creek and Gordon Creek enter the Sandy River (Figures 17 and 18). Aerial photography and terrestrial LIDAR data for 2006, 2007 and 2008 were combined with topographic surveys conducted during the summers of 2007, 2008 and In November 2006, there was a large storm in the Sandy Basin with a peak discharge of 41,152 cfs (7.8 year recurrence interval; Figure 19). Fishermen and park personnel report large changes at the mouth of Gordon Creek (lower right in Figure 17). The post-2006 channel change is apparent in the surveyed waterline and the photographed waterline (with only a 240 cfs difference in flow). By July 2007, the cutoff channel had deepened so that a flows greater than two feet were maintained even during low flows (<500 cfs). This cutoff channel isolated the former tip of the point bar and turned it into a mid-channel bar. Repeat surveys from 2006 and 2007 showed that the 2007 cutoff channel was up to 10 feet lower than the 2006 surface, while the bar surface had aggraded nearly 5 feet. There was little change between the summer of 2007 and 2008 following the dam breach. There were two large storm events during the winter of ; on November 13 th 2008, peak discharge was 30,929 cfs ( 3 yr recurrence interval), and on January 2, 2009, discharge peaked at 55,250 cfs (approx 20 year recurrence interval). These flow events created a second cutoff channel in the point bar and translated much of the sediment from the avulsion onto the downstream end of the point bar. 25

15 Stop 3: Oxbow Park Old Maid lahar deposits Recent channel change Figure 12 - Overview of Oxbow Park stops 26

16 References Stop 3b: Oxbow Park upstream (current channel changes) Figure 13 - Bank lines through time at the upstream end of Oxbow Park 27

References July 25, 2007 540 cfs January 24, 2008 1560 cfs July 26, 2008 834 cfs November 18, 2008

17 References July 25, cfs January 24, cfs July 26, cfs November 18, cfs May 20, ,500 cfs Figure 14 - repeat panoramic photos at the upstream end of Oxbow Park 28

18 References Figure 15 - Hydrograph for the Sandy River at Bull Run - just above Oxbow Par 29

Science on the Sandy River: Where is all the (Marmot) Dam sediment? Part II

Science on the Sandy River: Where is all the (Marmot) Dam sediment? Part II Johns Hopkins University National Center for Earth-surface Dynamics (NCED) Graham Matthews & Associates (GMA) Introduction Russ