Effects Of Dam Removal On Macrobenthos In The Elwha Nearshore

Abstract

The Elwha River dam removal project is expected to deliver fine grained sediment to the Elwha nearshore environment, thus potentially changing the substrate and affecting the macrobenthic invertebrate that inhabit it. This study looks at the relationship between grain size and macrobenthos. Grab samples were taken in April 2013 to evaluate for grain size and identify any invertebrates that were also picked up in the grab. The results were compared to those found prior to dam removal (2008) and that found in a similar study done spring 2012. Just outside of the river mouth, there were no benthos found and moving seaward numbers increased. Mud was found just outside the river mouth and to the west, and more sand was found in Freshwater Bay than had been prior to dam removal. Farther from the river mouth and to the east, there was mostly gravel and some sand, but little to no mud. It appears that in areas of higher gravel and sand fractions, there were more species found, as well as more total counts of benthos. In muddy areas there were little to no benthos found, which implies that the benthos are being smothered or migrating away from the areas with the most mud accumulation. This is an ongoing study and permanent effects on the benthic community are still unknown. However, this study concludes that during dam removal, the release of fine sediments will initially lower the presence of benthos near the river mouth.