Coupled Upstream-Downstream Geomorphic Responses To Deep Reservoir Drawdowns At A Fall Creek Dam, Oregon

The operation of 13 USACE high-head dams in the Willamette River Basin, Oregon, impacts important cultural, biological, and other natural or economic resources both upstream and downstream of the dams. While providing societal benefits, these dams also disrupt geomorphic processes and influence aquatic and riparian habitats and associated species , cultural sites and artifacts, and infrastructure. Although the dams primarily are operated to control flooding, other authorized purposes include water quality improvement, irrigation, fish and wildlife habitat, and recreation, all of which result in several meters of annual lake level fluctuations. Modifications to dam operations may alter reservoir and downstream conditions and processes; therefore, understanding coupled upstream-downstream interactions will aid managers in their assessment of overlapping priorities.

Since 2011, deep reservoir drawdowns at Fall Creek Lake, Oregon that lower lake levels to the historical streambed elevation were implemented to facilitate downstream passage of ESA-listed juvenile spring Chinook salmon through the 55-meter-high dam. Temporarily lowering the lake has increased the mobilization and downstream transport of predominantly sand and finer sediment to the lower gravel-bed reaches of Fall Creek and the Middle Fork Willamette River.

Upstream of the Fall Creek Dam, reservoir characteristics, streamflows, weather conditions, and dam operations during a particular drawdown control the magnitude of reservoir sediment erosion. Reservoir mapping shows that well-defined channels for the two main tributaries allow efficient conveyance of water and sediment through the reservoir during low lake levels, despite over 50 years of sediment accumulation since dam construction. During streambed drawdowns, fluvial erosion of reservoir deposits delivers mostly fine-grained sediment to the regulating outlets. Downstream of Fall Creek Dam, local deposition of fine-grained reservoir sediment in some low-velocity, off-channel areas along Fall Creek can be substantial, and cause reductions in wetted area and depth that lead to subsequent colonization and stabilization by vegetation. This stabilization reduces the likelihood of erosion and return to pre-drawdown conditions during future high flows. Loss of off-channel aquatic habitat is greatest along Fall Creek where drawdown sediment supply is high and the mixed bedrock–alluvial channel has few off-channel features. Downstream of the confluence with the larger Middle Fork Willamette River, sediment loads are diluted by increased discharge. Compared with Fall Creek, the Middle Fork Willamette River is a wider, alluvial river with larger and more numerous off-channel areas to accommodate fine-sediment deposition. Here, geomorphic changes are most apparent where reservoir sediment has accumulated in large eddies. However, direct linkages between drawdown operations and off-channel deposition in the Middle Fork Willamette River are challenging to establish.

While the underlying purpose for drawdown operations and geomorphic responses at Fall Creek may differ from other reservoirs in the Willamette River Basin, this information can be used to inform operations at other reservoirs to support management for reservoir sustainability. Although Willamette reservoirs share broad-scale hydrogeomorphic processes, controls, and resultant landforms, each reservoir contains geomorphically distinct expressions of local valley topography, streamflows, sediment inputs, shoreline erosion, and dam operations.