NEW INSIGHTS INTO THE GEOMORPHIC RESPONSE OF RIVERS TO DAM REMOVAL FROM FIELD AND LABORATORY STUDIES

About 75,000 irrigation, flood control, and hydropower dams in the United States are aging, deteriorating, or have outlived their useful lives and purposes. Not surprisingly, dam removal is emerging as both a challenge and opportunity for river management and research. Scientists at the Pacific Northwest Research Station in Corvallis, Oregon, are using scale models and monitoring actual dam removals to predict the responses of rivers to various dam removal scenarios. Of particular concern is the fate of sediments that have accumulated behind the dams. Reservoirs created by small dams are often completely filled with sediment and no longer store water. In these situations, the dam can be removed in one stage with only moderate impacts downriver. In contrast, reservoirs behind large dams typically still store water and are only partially filled with sediment. For this reason, large dams must be removed slowly by progressively notching the top of the dam. Through this method, the volume and quality of sediment released can be controlled, or at least predicted. Information generated by this research is being used to guide the dam removal process for two high-visibility projects—one on the Sandy River in Oregon scheduled for 2007 and another on the Elwha River in Washington scheduled for 2008. The experience gained through these and other removals will be used to develop pre-removal monitoring protocols for dam removals throughout the United States. As larger dam removals are carried out, opportunities arise to learn how rivers erode and digest sediment that has been stored behind the nation's many dams, and the consequences for downstream resources.