As the Elwha River’s two fish-blocking dams come down, the Lower Elwha Klallam Tribe will monitor how a portion of 20 million cubic yards of built-up sediment will affect the river’s estuaries at its mouth.

For the past century, sediment has built up behind the 108-foot-tall Elwha and 210-foot-tall Glines Canyon dams. As the dams are deconstructed the next few years, much of that sediment will come down the river.

“We know a lot of sediment will be deposited within the streambed of the lower river but we don’t know how much will be deposited in the estuary and how it will change,” said Matt Beirne, the tribe’s environmental coordinator.

Estuaries are important for salmon because they provide a place for salmon to hide from predators and to feed and grow before heading to sea.

The tribe has set up 15 sediment monitoring locations throughout the 50-acre estuary. Using a specialized measuring device called a Surface Elevation Table (SET), the tribe will determine at regular intervals how much sediment is building up throughout the estuary.

Lower Elwha Klallam Tribe natural resources staff use the SET to measure sediment in the estuaries at the mouth of the Elwha River.

The SET uses fiberglass pins to measure the change in height of the deposited sediment. This technique, developed by USGS researchers, has been applied to estuaries throughout the U.S. and internationally.

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

The tribe also will study how increased turbidity and sedimentation in the estuary may affect juvenile salmonids as the dam removal project progresses.

The dams are owned by the federal government and the Olympic National Park is spearheading the removal effort. The total cost of the project is estimated at $325 million.