RESERVOIR–DOWNSTREAM SEDIMENT BALANCE FROM ANNUAL DRAWDOWNS AT FALL CREEK LAKE, OREGON

A sediment budget developed from historical surveys, recent lidar, structure-from-motion datasets, and sediment gaging quantifies changes in reservoir sediment storage and associated downstream morphological responses resulting from annual drawdowns at Fall Creek Lake, Oregon. During the annual autumn drawdowns, the lake is lowered to streambed elevation to facilitate downstream fish passage through the 55-m high dam, which increases transport of predominantly fine (<2 mm) sediment to the lower gravel bed reaches of Fall Creek and the Middle Fork Willamette River. Preliminary results indicate a net sediment decrease of ~200,000 m³ within the lower reservoir between Water Years (WY) 2012–17. Downstream of Fall Creek Dam, patterns of sediment transport and deposition have changed over the five-year analysis period. Just downstream of the dam, suspended sediment loads computed from turbidity surrogates during the drawdowns have decreased from >50,000 tonnes in WY13 to just over 10,000 tonnes in WY17. Following the WY17 drawdown, three bedload measurements at the site had low discharge rates (<2.4 kg/s), but show an inverse trend in measured transport rate with discharge over time which reflects an exhausted, finite supply of coarse sediment released from the reservoir. Over the course of the drawdowns, morphological responses include a shift from coarse sands to fine sand and silt deposition in side channels along Fall Creek, and a decrease in annual sediment accumulation. Most off-channel aggradation measured from repeat lidar (2012 and 2015) was <0.6 m, with local maximums as much as 2 m. The main channels of Fall Creek and Middle Fork appear to have sufficient transport capacity to preclude long-term bed aggradation and effectively transport most reservoir sediments to the mainstem Willamette River. Off-channel areas, however, can accumulate and retain large quantities of sediment. Overall changes in the reach-scale sediment budget indicate that under continued drawdown operations, sediment evacuation from Fall Creek Lake will increasingly consist of annual suspended sediment influx from upstream reaches with limited local deposition in reaches downstream of the dam.