By Doug Garletts
Aquatic Stewardship Section, Willamette Valley Project
Each year, juvenile spring Chinook salmon protected under the Endangered Species Act must travel through Fall Creek Dam east of Eugene, Ore., on their way to the Pacific Ocean, where they eventually mature into adults and return inland to spawn in their natal streams.
The Corps has usually held Fall Creek Reservoir at a minimum elevation of 728 feet above sea level for flood damage reduction during the rainy winter season. Unfortunately, juvenile fish prefer to swim near the surface, and at that elevation they have a hard time finding a route through the dam due to the depth they must dive. In addition, when they do find a route, many are injured due to harsh passage conditions through the dam structure.
To increase juvenile passage and survival through Fall Creek Dam, the U.S. Army Corps of Engineers in 2011 lowered the elevation of the reservoir to 680 feet. Data has shown that lowering the reservoir pool to that level during juvenile migration results in roughly a ten-fold increase in the numbers of adult salmon that later return to Fall Creek compared to holding the pool at 728 feet.
The Corps completed Fall Creek Dam in 1965. For over 45 years, the calm waters of the reservoir have allowed for large accumulations of fine sediment as well as coarse material like wood, sand and gravel. Without the dam, this material would have travelled downstream, providing a natural source of enriching nutrients and spawning gravels to the lower reaches of Fall Creek and the Middle Fork Willamette River.
Lowering the reservoir to near historic creek bed elevations has allowed this material to be moved downriver again. Plankton, aquatic plants and insects, and larger vertebrates such as fish and mammals all benefit from the cycling of nutrient-rich waters. The entire food web downstream of the reservoir will see a great benefit over time from the liberation of the trapped material.
As the reservoir begins to fill in January, the sediment-rich reservoir bottom will once again covered by calm water and turbidity downstream will drastically lessen. The creek channel will reshape itself into a more natural, dynamic system similar to what it once was pre-dam. The natural cycle of renewal will turn once again.