Benthic invertebrate and periphyton assemblages in floodplain habitats above, between, and below the Elwha River dams: Forecasting response to dam removal

The removal of two dams on the Elwha River in western Washington will be one of the largest dam removal efforts ever undertaken.  While the effects of dams on aquatic ecosystems have been well documented, the effects of dam removal have not. Those studies that do exist are primarily concerned with the removal of small dams (< 30 m in height), and in watersheds without historically large populations of anadromous fish.  The long-term goal of this study is to evaluate how periphyton and benthic invertebrate assemblages respond to dam removal above, between, and below the Elwha dams. In order to detect such future changes, our pre-dam removal monitoring objectives are to (1) document baseline conditions, (2) examine existing differences between the three reaches of the river, and (3) to establish long-term monitoring stations across the Elwha and a neighboring reference basin. Our preliminary data indicate that significant differences do exist in periphyton biomass and invertebrate taxonomic composition between dammed and un-dammed reaches of the Elwha River, but that these differences are less extreme than often seen in regulated systems.