EVOLVING FLUVIAL RESPONSE OF THE SANDY RIVER, OREGON, FOLLOWING REMOVAL OF MARMOT DAM

The October 2007 removal of Marmot Dam on the Sandy River, Oregon, triggered a rapid sequence of fluvial responses as ~730,000 m 3 of sand and gravel that filled the former reservoir were suddenly exposed to an energetic river. Using direct measurements of sediment transport, photogrammetry, and repeat surveys between transport events, we monitored the erosion, transport, and redeposition of this sediment in the hours, days, and months following breaching. Measurements of suspended load and bedload documented an initial spike in the flux of fine suspended sediment in the minutes after breaching followed by high rates of suspended- and bedload transport of sand. Significant gravel transport did not begin at a measurement site 0.4 km downstream of the dam until 18-20 hours after breaching, when bedload transport achieved rates of about 60 kg/s—rates that greatly exceeded concurrent measurements of less than 10 kg/s at sites upstream and farther downstream of the dam. Bedload transport rates just below the dam site remained 10-100 times above upstream and downstream rates through subsequent high flow events during the winter and spring of 2007 and 2008. Much of the elevated sediment load was derived from eroded reservoir sediment, which initially began eroding when a multi-meter-tall knickpoint migrated upstream 200 meters in the first hour.