Fall Creek Drawdown

Science Center Objects

Each autumn Fall Creek Lake is drawn down to allow endangered juvenile salmonids to pass freely through the dam. The drawdowns involve lowering the lake water level to the lake bed, creating a fluvial environment characterized by large amounts of sediment being transported through the dam and into Fall Creek and the Middle Fork Willamette River.

The U.S. Army Corps of Engineers (USACE) is an Action Agency listed in the 2008 National Marine Fisheries Service (NMFS) and U.S. Fish and Wildlife Service (USFWS) Biological Opinions (BiOps) on continued operations of the Willamette Valley Project. The 2008 BiOps proposed a variety of actions to improve conditions in the Willamette Basin for Endangered Species Act (ESA)-listed fish species. The BiOps state that the Action Agencies carry out interim operational measures to pass downstream migrants as safely and efficiently as possible until permanent downstream fish passage facilities are constructed. In response, USACE proposed a short term study on modifying the operation of Fall Creek Reservoir to improve passage for these downstream migrants. The modification involves lowering the December elevation of the Reservoir to 680 feet, 48 feet below the normal winter low-pool elevation of 728 feet. The reservoir will remain at an elevation of 680 feet for approximately 2 weeks. Lowering the reservoir to this elevation would potentially allow juvenile fish to pass through the regulating outlets and diversion tunnel as opposed to the generator turbines, where mortality rates are high.

USACE carried out a similar drawdown at Fall Creek Reservoir in 2011 and 2012 to investigate drawdown effects on downstream passage of juvenile salmonids. This operational change was considered a success due to the passage of fish quantified by screw traps, but also resulted in the release of large amounts of sediment to Fall Creek below Fall Creek Dam. The currently proposed drawdown is likely to also liberate large amounts of sediment from the exposed upper reaches of the reservoir pool, increasing turbidity and sediment transport in Fall Creek and the Middle Fork Willamette River.

Suspended solids in aquatic systems consist of both organic and inorganic matter such as clays, silts, finely divided organic matter, plankton, and other microscopic organisms, organic acids, and dyes. High concentrations of suspended solids in aquatic systems can lead to alterations in physical, chemical, and biological properties of water bodies, adversely impact salmonid spawning habitat and water quality, and increase costs of water treatment. Fine grained suspended sediment deposited in spawning redds can have lethal and sublethal effects on salmonid eggs that incubate within the gravel voids. Additionally, bedload transport that results in streambed scour and fill can also result in high mortality rates of incubating salmonids. Because of the detrimental effects of mobilizing sediment due to reservoir drawdown, adequate monitoring of suspended sediment, bedload, and water quality above and below Fall Creek Reservoir is warranted. This study will provide data to USACE that will allow the Agency to determine if this annual operational modification is appropriate to provide downstream fish passage at Fall Creek Dam.