IMMEDIATE DOWNSTREAM HYDROLOGIC AND GEOMORPHIC RESPONSE TO THE CONDIT DAM REMOVAL, WHITE SALMON RIVER, WASHINGTON

The 26 October 2011 breach of Condit Dam on the White Salmon River, Washington, was one of the largest and most dramatic decommissionings so far in an ever-more common tactic for river restoration. The 38-meter-high concrete structure, built in 1913, impounded a 37 hectare reservoir partly filled with 1.8 million cubic meters of sand, mud, and gravel. Rapid evacuation of the reservoir water—by explosively creating a 6-meter-diameter hole at the bottom of the dam—was followed by rapid incision and mass mobilization of reservoir sediment. At the USGS streamflow measurement station 2.33 kilometers downstream from the dam, discharge rose from 1.5 cubic meters per second (m³/s) prior to breaching to about 400 m³/s 20 minutes after breaching and 11 minutes after arrival of the flow front. Sediment concentrations continued to rise as discharge diminished, attaining a peak value of more than 0.7 kilograms per liter 1.5 hours after the breach as the initial water flood transformed into a mudflow in conjunction with near complete water drainage from the reservoir. The passing flow left massive to stratified sand and mud deposits flanking the channel. From these measurements, we can link sediment concentrations, flow characteristics and deposit sedimentology for the day of the breach. Continued sand and, later, gravel transport led to >1 meter of channel filling at the gage site during the ten days following the breach, but this fill was incised to nearly pre-breach levels five days later as the wave of sediment moved downstream leaving sand and gravel deposits flanking the channel for the first time in decades.