Monday, April 22, 2013

Elwha River Fine Sediment Sampling

Late last summer, the U.S. Fish and Wildlife Service (FWS), NOAA Fisheries, Lower Elwha Klallam Tribe, U.S. Geological Survey, and National Park Service participated in a multi-agency effort to measure fine sediment concentrations in salmon spawning areas of the Elwha River. This sampling was the first to occur following the initiation of dam removal on the Elwha River; baseline data had previously been collected for 2 years prior to dam removal.

Sampling area within the 'shield' showing
sediment conditions prior to dam removal (2010)

The removal of Elwha and Glines Canyon Dams is expected to release 7-8 million cubic meters of sediment. About half of this is fine sediment (silt and clay), which should be transported quickly by the Elwha River into the Strait of Juan de Fuca. However, this material could alter spawning habitat downstream of the dams. Monitoring fine sediments in locations where salmon are likely to spawn will help determine whether any impacts occur and, if so, how long they persist.

We sampled both mainstem and floodplain channel habitat, collecting 30 samples from just below Glines Canyon Dam to near the mouth of the river. A plywood shield blocked water flow during sampling, providing a calm water area where the sample of the river bed material could be collected. We removed the surface layer of the sediment and placed it into a sample bag for later processing to determine sediment size distribution. We also collected a 'before-and-after' water sample to determine the amount of fine sediment that was suspended in the water column during sampling.

Measuring pre-sampling water depths within the shield.
This is done to determine the depth of the sediment sample and
the water volume behind the shield. This volume is used to
calculate the overall weight of fine sediment suspended
in the water column during sampling.
Preliminary results suggest that there is more fine sediment in the spawning areas today than before dam removal began; however, the level was lower than expected. There was also a change from large cobble to gravel substrate in some areas, which will greatly improve salmon spawning habitat. We expect larger changes this year following the complete removal of Glines Canyon Dam, which holds back the majority of the sediment in this river system.


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