Liberated rivers: lessons from 40 years of dam removal

In recent decades, dam removal has emerged as a viable national and international strategy for river restoration. According to American Rivers, a river conservation organization, more than 1,100 dams have been removed in the United States in the past 40 years, and more than half of these were demolished in the past decade. This trend is likely to continue as dams age, no longer serve useful purposes, or limit ecological functions. Factors such as dam size, landscape and channel features, and reservoir sediment characteristics differ widely, so dam removal projects must be evaluated individually to determine the best approach. Stakeholders need trusted empirical findings to help them make critical decisions about removal methods, how to recognize and avoid potential problems, and what to expect in terms of geomorphic and ecological recovery.

Gordon Grant, a research hydrologist with the Pacific Northwest Research Station, in partnership with a U.S. Geological Survey working group, extracted key lessons from studies of dam removals to help guide future removals and predict geomorphic and ecological outcomes. The combined findings provide evidence that rivers are remarkably resilient, and when dam removal is well planned and executed, recovery is swift and few long-term problems have occurred. Although geomorphic responses are reasonably predictable, biological responses are less so, and in both cases dam owners and project managers would do well to plan for the unexpected. Numeric and physical models are proving to be valuable decisionmaking tools.