Changes to dam operations yield strong wild salmon returns

Winter 2012

Fall Creek, located in Oregon’s Willamette River basin, is home to some of the region’s richest salmon habitat. Nearly 200 square miles drain into the stream and feed the Middle Fork of the Willamette River just south of Eugene, Oregon. In 1965, the U.S. Army Corps of Engineers built Fall Creek Dam to protect communities, like Eugene, from the torrents of annual floods. Since its construction, the dam has prevented more than $2.5 billion in potential flood damages. The dam also has prevented spring Chinook salmon from reaching their once abundant, and still highly productive, habitat at the base of the Cascade Mountains—until now.

In 2008, NOAA Fisheries issued a Biological Opinion to guide the Corps of Engineers’ operations of the Federal flood control dams in the Willamette River basin in a manner that protects listed salmon and steelhead. This includes spring Chinook that migrate to and from Fall Creek Dam, but are impeded by the blockage from spawning and rearing above the dam. Some may question whether we can in fact restore salmon and steelhead above high head dams with huge reservoirs like Fall Creek. If we help adult fish pass above the dam and provide their offspring with safe downstream passage through the reservoir and dam, are they adaptable and resilient enough to survive? The Corps’ recent operational changes at Fall Creek Dam, as guided by NOAA Fisheries’ Biological Opinion, suggest that fish not only survive, but thrive.

The Corps of Engineers passes migrating adult spring Chinook salmon upstream via a trap located at the base of Fall Creek Dam—fish are guided into the structure, loaded onto a truck, and released into habitat upstream of Fall Creek Reservoir. Prior to 2009, the Corps of Engineers passed both wild and hatchery fish above the dam. Wild and hatchery fish seeded the vacant habitat that was lost for several generations. These same fish reproduced upstream of the dam and reservoir, their offspring migrated downstream through the reservoir and dam, utilized habitat in the mainstem Willamette River, journeyed onto the ocean, and came back as healthy adults to be passed above the dam. This started the life cycle once again.

However, in 2009 the Corps of Engineers altered its traditional program and only released wild adults above the dam. The Corps also modified its reservoir operations, drawing it down earlier and lower in the fall to help young salmon pass through the reservoir and the regulating outlets of the dam. The fall drawdown does increase turbidity levels in reaches below Fall Creek Dam, raising concerns about the effects this may cause to salmon. The temporary rise in turbidity levels, however, is not likely to harm more than a few rearing and migrating fish; whereas thousands of juvenile Chinook will benefit for the operational changes and pass quickly and safely through the reservoir and dam. The number of returning wild fish speaks for itself, with 330 to 550 wild fish returning since the change in operations. These wild fish represent a significant portion of the current wild fish production in the Willamette River basin. By making strategic and minor changes to dam operations, wild spring Chinook have rebounded in some of the basin’s most pristine habitat.

The wild Chinook returns to Fall Creek demonstrate that if we provide fish with productive habitat and facilitate downstream passage through the reservoir and dam, many will survive and return in meaningful numbers. Wild fish will seed the pristine habitat that awaits them if we simply give them the opportunity to use it.

Photographs:

Main page: Pristine habitat above the Fall Creek dam. Photo by John McMillian, NOAA Northwest Fisheries Science Center

This page: Fall Creek Dam. Photo courtesy the Army Corps of Engineers