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Willamette Valley Project
541-684-4300

cenwp-pa@usace.army.mil
503-808-4510

Fall Creek Deep Drawdown

The Corps annually draws down Fall Creek Reservoir to help move juvenile fish past the dam. During the 2015 drawdown, Corps contractors constructed new intakes to supply water to an existing fish ladder.

Problem

Each year, endangered juvenile spring Chinook salmon must travel through Fall Creek Dam on their way to the Pacific Ocean, where they eventually mature into adults and then 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 – 50 feet or more - to reach the dam’s regulating outlets. In addition, when they do find a route, many are injured due to harsh passage conditions through the dam structure. 

The calm waters of the reservoir have also allowed for large accumulations of sediment, wood, sand and gravel.  Without the dam, this material would have traveled downstream, providing a natural source of enriching nutrients and spawning gravels for the lower reaches of Fall Creek and the Middle Fork Willamette River.

Solution

To increase juvenile passage and survival through Fall Creek Dam, the U.S. Army Corps of Engineers in 2011 began lowering the elevation of the reservoir for a short period in November or December to 680 feet – only 10 feet above the regulating outlet entrance and near the historical streambed.

Results

Data has shown that lowering the reservoir pool during juvenile migration results in roughly a ten-fold increase in the adult salmon that later return to Fall Creek compared to holding the pool at 728 feet.

It has also allowed material trapped behind the dam to be moved downriver. 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.

Downstream fish passage through Fall Creek - Deep Drawdowns (2015)