Impacts of Dams on Flow Regimes in Three Headwater Subbasins of the Columbia River Basin, United States

We compared long-term changes in flow regimes resulting from climate change with those resulting from dams in three matched pairs of natural and modified headwater subbasins of the Columbia River. Based on the analysis of 12 flow-regime metrics, we found that damming had minimal effect on most quantity of flow metrics, but major effect on timing of flow metrics, especially those representing "spring runoff." In all modified subbasins, "spring runoff" metrics occurred much earlier than natural flow (up to ~44days earlier for April-July flows). Storage capacity modulated the magnitude of timing of flow-metric changes, with the largest storage capacity leading to the most change. However, even in subbasins with low storage capacity, we found significant change in most timing of flow metrics. We also found that damming, especially in subbasins with higher storage capacity, overwhelmed climate variability in all basins for most flow metrics. This shows that reservoir operations need to be modified to more closely match the natural timing of flow regimes to promote positive ecologic response in modified rivers, even in basins where quantity of flow metrics have not changed substantially as a result of damming.