The removal of the two Elwha river dams in the US would increase siltation and erosion in the reach of the river downstream of the dams, according to a new US Geological Survey (USGS) report.

The USGS study, based on a 1994 experiment, does not fully predict what would happen once the dams are removed. However, it says about 230,000m3 of sediment was released when the level of Lake Mills, behind the Upper Glines Canyon dam, was dropped by 5.5m during the week-long experiment six years ago.

The Elwha dams were acquired by the federal government earlier this year for US$29.5M from their owners and operators, the Fort James Paper Company and Daishowa America, under the Elwha River Ecosystem and Fisheries Restoration Act of 1992. The 1992 Act authorises the interior secretary to fully restore the river’s ecosystem and native fisheries. Opponents of dam removal have said setting free seven decades worth of backed-up sediment would raise the river level.

Currently both dams are continuing to produce power. They are now part of the regional power grid over- seen by the Bonneville Power Administration.