Wood Debris Sources and Logjam Distribution and Dynamics Before and After Two Large Dam Removals on the Elwha River, WA
Abstract
Logjam distribution and dynamics before, during, and after two large dam removals on the Elwha River, WA are measured through aerial photographs and field surveys. Prior to dam removal, local wood recruitment dominated logjam dynamics through bank erosion, channel migration and avulsions. Several large logjams formed in side channels during floods, and gradually revegetated, producing new floodplain forest. Removal of the Elwha and Glines Canyon dams resulted in increased downstream flux of sediment and wood from the upstream watershed and material that was previously stored in the reservoirs behind the dams. During the first year of dam removal, 208 pieces of wood debris were individually tagged in the former Lake Mills, the reservoir behind the Glines Canyon Dam, and the upper part of the Middle Reach (between the dams). After dam removal, 62 tagged pieces were relocated, showing wood transport out of Mills into the Middle Reach, with a few pieces that traveled to the former Lake Aldwell, the reservoir behind the Elwha Dam, and the Lower Reach (between the Elwha Dam and the coast). During and immediately after dam removal, logjam area increased in the channel main-stem, logjams developed unexpectedly rapidly in the former Lake Aldwell reservoir, and very little logjam development occurred in the former Lake Mills reservoir. In the river channels between Aldwell and Mills (Middle Reach), logjam area increased with increased upstream wood flux and more local recruitment as the higher sediment load increased lateral channel mobility. However, logjam development in the two former reservoirs followed very different trajectories due to rapid erosion of sediment stored in the former Aldwell reservoir, a greater upstream flux of wood into Aldwell, and the presence of rooted 1 -3 m diameter tree stumps in the Aldwell sediments that became anchor points for logjam formation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP41C2349L
- Keywords:
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- 0442 Estuarine and nearshore processes;
- BIOGEOSCIENCES;
- 0481 Restoration;
- BIOGEOSCIENCES;
- 1813 Eco-hydrology;
- HYDROLOGY;
- 1825 Geomorphology: fluvial;
- HYDROLOGY