Upstream Geomorphic Response to Dam Removal: The Blackfoot River, Montana
As dam removal is increasingly used as a tool to restore rivers, developing a conceptual and field-based understanding of the upstream fluvial response is critical. Using empirical data and modeling, I investigated the spatial and temporal pattern of reservoir sediment erosion and upstream channel evolution of the Blackfoot River, MT, following the 8 m base level reduction caused by the removal of Milltown Dam. Field data collected include surveys of channel bed topography and water surface elevation profiles which were integrated into a flow modeling approach. Headward erosion extended 4.5 km upstream of the dam site during the first five months following the dam removal. In the lower 1.8 km of the reservoir, up to 3 m of highly mobile silt and sand was evacuated. Upstream, the river incised into a coarse deltaic sediment deposit (D50 70mm) in the upper reservoir. The analysis of erosion through the hydrograph shows that the channel incised up to 2 m in some locations and maximum volumetric erosion of 260,000 m3 was reached several days after the flood peak (286 m3/s, 3.5 year return interval). Net erosion following the dam removal, accounting for both scour and deposition, was 150,000 m3 across the 5 km study reach. The modeling-based water surface elevation analysis revealed the intra-hydrograph pattern of erosion that otherwise would have been missed by comparing pre- and post-removal cross section topography. The post-removal evolution of the lower Blackfoot was heavily influenced by confinement of the channel and the above average discharge. Widening was associated with areas of local aggradation, whereas narrowing was associated with degradation—a finding similar to those from previous flume experiments.
base level, dam removal, large woody debris, reservoir sediment erosion, upstream geomorphic response
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