River Response To Dam Removal: A Synthesis Perspective
East, Amy E 1
1 U.S. Geological Survey
The last several decades have seen a substantial increase in the number of dams removed in the U.S., including the recent removal of large dams impounding millions of cubic meters of sediment. Both dam emplacement, and eventual dam removal, can be large disturbances to rivers’ physical and ecological settings and processes. Interpreting river response to these disturbances is essential to understanding fundamental earth-surface processes, as well as human effects on rivers and surrounding landscapes.
From the recent wave of dam-removal studies, common findings emerge: (1) Rivers are resilient, showing rapid geomorphic and sedimentary response to dam removals, especially when removals are sudden rather than prolonged. Rivers can rapidly evacuate large percentages of stored reservoir sediment (?40% within one year)—particularly where sediment is coarse-grained (sand and gravel), and can move the evacuated sediment long distances. The channel downstream typically takes months to years—not decades—to achieve a degree of stability within its natural range of variability. (2) Modest flows can erode large amounts of reservoir sediment; large floods are not required to move substantial sediment volumes. Portions of the redistributed reservoir sediment remain (up- and downstream of the dam site), shaping a new landscape. (3) Dam height, sediment volume, and sediment grain size and cohesion strongly influence response to dam removal. Although removals of small dams with little stored sediment are more common, removals of large dams (?10 m) with major sediment releases have had longer-lasting and more widespread downstream effects. (4) Downstream valley morphology and hydrology influence the distribution of released sediment.
Geomorphic and water-quality changes during and after dam removal affect riparian ecosystems in myriad ways. Dam removal restores longitudinal connectivity, removes migration barriers for fish and other organisms, modifies habitat structure through sediment transport and deposition, and leads to shifts in biotic community structure. Longer-term, comprehensive physical and ecological studies from a broader biogeographic range will be valuable in coming years to test hypotheses about rates of recovery, resilience, and the effectiveness of dam removal as a landscape- and ecosystem-restoration practice.