Tribe, USGS diving for sediment in Elwha River delta and Strait

The dramatic sediment plume forming at the mouth of the Elwha River the past year has tribal and federal scuba divers looking for changes to the marine habitat.

The plume is the result of thousands of cubic yards of sediment that have been unleashed from behind the river’s Elwha and Glines Canyon dams, which have been undergoing deconstruction since September 2011.

Divers from the Lower Elwha Klallam Tribe and U.S. Geological Survey have been surveying the nearby shoreline and deep waters to observe immediate changes to marine life.

“It appears most of the fine sediments are not depositing in the subtidal survey areas just yet,” said Matt Beirne, the tribe’s environmental quality manager. “A couple of our divers recently swam up the mouth of the river and noted some significant sand deposition. What could be happening is that the sand is depositing at the river mouth but the finer sediments are being carried out a greater distance. I think we may see some major changes this winter when the high river flows from seasonal flood events resume.”

The dams prevented the river from regularly flushing sediment from the upper watershed. Today, the beaches by the mouth of the river are dominated by cobbles, instead of a sandier environment that tribal elders remember, Beirne said. Tribal members hope that a return of finer sediments in the nearshore will create soft bottom habitats that will spark the return of clams and other shellfish.

Beirne also is watching how the estuary at the river’s mouth is being impacted by the sediment. Tribal staff installed 15 measuring stations throughout the estuary last year, which will show how much sediment settles over time.

“We’re not seeing any significant changes yet but there are a few places where a couple inches of material have been deposited,” he said. “I would expect to see some more significant deposition this fall as we see more flood events in the river.”

The tribe also has deployed water quality sensors to collect data on water clarity (turbidity), salinity and temperature in the estuaries. This will show how the sediment-laden water will be distributed throughout the river mouth on tidal, seasonal, and annual cycles.

The tribe also will resume studies in Spring 2013 that examine fish populations and how increased turbidity and sedimentation in the estuary may affect juvenile salmonids and their diet as the dam removal project progresses.