Elwha dam removals effect on the carbon cycle: tracking particulate organic carbon in the nearshore

Abstract

Dam removal is an emerging technique for ecosystem management. Reservoirs behind the dams are known to be both carbon sinks and sources. The Elwha dam removals in Washington State are the largest dam removals in U.S. history. In this study the fate of the carbon stored in the reservoirs upon its abrupt release is evaluated. Sediment samples from the estuary, delta and water column were tested for their particulate organic carbon content through loss-on-ignition testing. Grain sizes of sediment samples were analyzed and volume estimates of carbon deposits were calculated. The results indicated that particulate organic carbon was preferentially deposited in two locations. The large woody debris typically settled on the subaerial delta, while the carbon sorbed to silt and clay settled in the estuary and subaqueous delta. The sudden release of the reservoirs resulted in a condition where river meandering through the former reservoir beds is minimal. As a result, approximately half of the carbon likely sorbed to the sediment in the reservoirs remains. The rapid sediment accumulation resulted in a higher-than-average carbon burial rate of the roughly 165,000 +/- 124,000 tonnes of carbon deposited on the delta. This suggests dam removal can not only restore an ecosystem and may also be able to help mitigate climate change.