Elwha River Dam Removal: Check out the mud in the Strait!
What will happen to the sediment from behind the dams once it reaches the marine environment?
Understanding the sedimentary processes is a critical aspect of assessing the impacts of the dam removal on the marine environment.
You have probably all read in the newspaper about the Elwha River dam removal that is in progress. The major story we hear concerning the purpose of the dam removal is about the restoration of salmon spawning grounds in the river, but an additional goal of the removal is to restore the delivery of river sediment to coastal environments in the Strait of Juan de Fuca. What will happen to this sediment once it reaches the marine environment? Understanding the marine sedimentary processes is a critical aspect of assessing the impacts of the dam removal on the marine environment.
Although the latest reports state that the river is now in its former channel, there is still a lot of sediment in the former reservoirs (over 18 million m3) that has only just begun to be moved by the river. We predict that during the spring snowmelt period and early winter storms, unprecedented conditions for sediment transport will be in place. When these sediment-laden flows reach the marine environment they will create a dramatic surface plume, and will likely form a hyperpycnal plume, i.e., a plunging freshwater plume that is denser than seawater. These flows will surge into the depths of the Strait, and are the focus of our research funded by NSF (with baseline studies supported by WA Sea Grant), as they are an analog for conditions that may occur with extreme floods around the world. Such floods occur on decadal time scales, and yet may dominate the stratigraphic record found in continental shelf deposits near small mountainous rivers.
A recent cruise on the R/V Barnes showed some of the first traces of sediment from the dam removal on the Elwha delta. Graduate student Emily Eidam is working with an undergraduate team in a Friday Harbor Research Apprenticeship program (http://depts.washington.edu/fhl/studentSpring2012.html) to analyze this initial data and quantify the seabed changes to date. No hyperpycnal plumes have occurred yet, but our instrumented tripods are deployed and ready to capture their dynamics, and a cruise planned for late June (coinciding with high river discharge during the spring snowmelt) will give us the opportunity to understand these elusive features.
We are also interested in the general dispersal of fine-grained sediment on the delta under more typical conditions. Understanding the combined set of transport conditions allows us to predict changes to the seabed due to short- and long-term dam removal changes to the sediment supply to the subaqueous delta. We are working with other groups (including USGS and WA Sea Grant) as part of the Nearshore Research Consortium (http://www.elwhainfo.org/research-and-science/nearshore-consortium) to understand the changes to nearshore habitats due to dam removal projects, on the Elwha River and other locations world wide.