The U.S. Geological Survey (USGS), in collaboration with the U.S. Army Corps of Engineers (USACE), investigated the effects of annual streambed drawdowns at Fall Creek Lake in west-central Oregon. Since 2011, Fall Creek Lake has undergone temporary drawdowns that fully drain the lake in the autumn to aid the downstream migration of juvenile spring Chinook salmon through the dam. While this practice benefits fish passage, it also triggers a cascade of interlinked geomorphic changes within the upstream reservoir and downstream reaches of Fall Creek and the Middle Fork Willamette River.

Draining the reservoir down to a free-flowing stream causes erosion within the lakebed and the temporary mobilization of sediments—mostly sand and finer grained—to the downstream reaches of Fall Creek and the Middle Fork Willamette River. During water years 2012–18, USGS monitored and analyzed how sediment, landforms, and stream channels shifted and changed both upstream and downstream of the dam in response to drawdowns. 

Result highlights:

Reservoir dynamics: The landscape, coupled with the dam's infrastructure and operational procedures, plays a pivotal role in determining the amount of sediment movement downstream. For example, the regulating outlets at Fall Creek Dam are at the bottom of the dam and allow the lake to convert to a river that can transport reservoir sediments downstream; however, other Willamette River Basin dams operated by USACE do not have the same infrastructure or capabilities. Land features within Fall Creek reservoir reflect a history of alternating depositional, transport, and erosional processes influenced by reservoir operations and pre-dam topography.

Sediment transport: Sediment monitoring downstream from the dam reveals that most sediment released from Fall Creek Lake consists of sand-sized particles or finer. Although suspended sediment loads varied from year to year, the overall seasonal patterns of transport throughout each year were similar during monitoring in water years 2013–2018, increasing rapidly during drawdown operations and declining over the following months.

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Downstream impacts: During streambed drawdowns, the upper segments of Fall Creek and the Middle Fork Willamette River initially become saturated with sediment. However, over the following weeks to months, the main channels clear the sediment, with low-velocity areas accumulating finer-grained sediment. In some places, especially low-velocity areas with existing vegetation, new vegetation can rapidly establish on newly deposited sediment, reinforcing stability. However, some of these patterns of deposition and vegetation establishment occur at small, localized areas and may not be detectable if monitoring at larger scales or shorter timeframes.

Application to other rivers and reservoirs: The approaches and findings from the Fall Creek Lake study provide a framework for understanding geomorphic responses to drawdowns in diverse contexts such as sediment management or construction projects elsewhere. This research is particularly useful amidst ongoing changes in dam operations across the Pacific Northwest, including court-ordered modifications at Willamette Valley reservoirs implemented in 2021. The approaches and findings can be used to help decision-makers and managers as they navigate the complex task of balancing multiple objectives, such as flood-risk management and fish passage, in dam management.

Changes to operations following court-ordered mandates have prompted additional monitoring beyond this study.

Future Directions: Continued monitoring is essential to address outstanding questions regarding the long-term geomorphic changes resulting from drawdown operations and effects on reservoir and river management issues (for example, vegetation colonization and changes to off-channel features that affect aquatic habitats, flood-risk mitigation, or cultural resources). This could involve a range of techniques, from sediment transport monitoring to repeat mapping and surveys to hydraulic modeling. However, findings from this study show that it is useful to use a combination of approaches at multiple scales in space and time.

For more information, see the report published on May 17, 2024, Geomorphic responses of Fall Creek and Middle Fork Willamette River to streambed drawdowns at Fall Creek Lake water years 2012–18.