Monitoring the effect of deep drawdowns of a flood control reservoir on sediment transport and dissolved oxygen, Fall Creek Lake, Oregon

Annual reservoir drawdowns at Fall Creek Lake, Oregon, have occurred for eight consecutive years from December 2012 to November 2019. The annual drawdowns are the result of the 2008 Biological Opinion of the US Army Corps of Engineers (USACE) Willamette Valley Project operations, which directed the USACE to carry out interim operational measures that would provide volitional downstream passage for endangered species act (ESA)-listed Chinook salmon. At Fall Creek Lake, the USACE modifies its operations by lowering the reservoir elevation to 690-ft, approximately 40 feet below the normal winter low-pool elevation. This action results in a runof-river scenario through the dam allowing juvenile Chinook salmon to safely pass through the regulating outlets. Monitoring of juvenile Chinook salmon in screw traps at the outlet of the dam has shown variable timing in out-migration associated with reservoir elevation, and that most of the juvenile fish exited the reservoir when the pool elevation passed 700-ft (Taylor and others, 2015). The annual drawdown has therefore been effective in providing safe downstream fish passage and has also had the collateral effect of transporting large quantities of suspended sediment to the downstream reaches of Fall Creek and the Middle Fork Willamette River. The US Geological Survey (USGS) has calculated time-series of suspended sediment concentrations (SSC) and suspended sediment loads (SSL) before, during, and after the drawdowns for six of the last nine drawdown years (water years [WY] 2013-2018), which have lasted between 5-14 days. The transport and deposition of sediment from the drawdowns has affected side-channel habitat below the dam by depositing large quantities of sand-size material resulting in streambed aggradation in several locations. The results from the USGS monitoring effort have provided important information to USACE on how the modification of their operations has affected sediment transport in the river reaches below the dam.