The Lower Elwha Klallam Tribe is taking a census of summer chinook and winter steelhead in the Elwha River before its two dams are removed in September by using a weir and a Dual Frequency Identification Sonar (DIDSON).
“Our aim is to establish a year-round counting station using the weir and DIDSON to evaluate salmon populations before and after dam removal,” said Keith Denton, a consultant to the tribe on the project.
For nearly 100 years, fish have been blocked from the upper Elwha River watershed by two dams, the 108-foot-tall Elwha Dam and the 210-foot-tall Glines Canyon Dam, which were built without fish ladders. Puget Sound chinook and steelhead are listed as “threatened” under the federal Endangered Species Act.
Developed at the University of Washington for the U.S. Navy to find mines on ship hulls, DIDSON was quickly applied to fisheries management. DIDSON also has been used to track juvenile salmon on the Columbia River and adult spawners in Alaska.
The DIDSON beams sound waves across the river, capturing images of fish that swim past. The weir is like a fence in the water, which enables the upriver bound fish to be collected for information such as species and size.
The weir is removed during periods of high water flow but the DIDSON can be used during almost all flow conditions, Denton said.
“Setting this system up prepares us for long-term monitoring of the river when the dams come down and we will see what’s going up into the upper watershed,” he said.