Studying Elwha River sediment is big part of dam removal prep

Standing in middle of the rushing lower Elwha River, Mike McHenry hefts a slippery rock, measures its widest point at about 10 inches, then heaves it back into the water.

Lower Elwha Klallam Tribe Habitat Manager Mike McHenry takes a measurement of a cobblestone found in the Elwha River.

“The whole lower river is filled with this type of large cobble and smaller boulders,” said the Lower Elwha Klallam Tribe habitat program manager. “This is not good for salmon spawning habitat—they need a mixture of smaller gravels for egg nests.”

Measuring rocks and collecting shovelfuls of other streambed material is part of the tribe’s latest pre-dam removal project. The fish-blocking Elwha and Glines Canyon dams are set to be removed beginning next September and will take 3 years to demolish. The project will restore salmon access up the river as far as the foothills of the Olympic Mountains, where salmon haven’t accessed for nearly 100 years.

The current project involves collecting sediment samples from riffle crests along a longitudinal gradient from the river mouth to near Glines Canyon dam, 10 miles upriver. During August, a total of 60 samples were collected at 20 different sites in the reach. The samples represent the current or pre-dam removal condition. Repeat samples will be taken following dam removal to assess the changes in spawning habitat for salmon.

“Baseline monitoring such as this is critical to understanding how large rivers will respond to dam removal, said George Pess, a National Oceanic and Atmospheric Administration’s (NOAA) Northwest Fisheries Science Center (NWFSC) Fisheries Research Scientist and project leader. “We have been working on such studies with the Elwha Tribe for a decade and see these as a critical part of assessing the benefits of dam removal.”

Tribal staff and biologists from the NWFSC filled bags with fist-sized rocks and sediment from 20 sites that could support salmon spawning if good gravel was available. The material collected will be examined and the information will be entered into a database about the pre-dam removal condition of the river sediment.

Once dam removal starts, biologists expect the streambed to change significantly, since there are 18 million cubic yards of sediment behind the two dams. While some of that sediment will be used to help rebuild the floodplains near each dam site, sediment is also expected to flow down river and alter the existing streambed.

The 108-foot Elwha Dam and the 210-foot Glines Canyon dam are owned by the federal government; the Olympic National Park is spearheading the removal effort. The project to remove the structures and restore the Elwha River ecosystem, estimated at $350 million, is the largest dam removal project to date in the United States.