COMPARISON OF PRE- AND POST-DAM ATTRIBUTES OF THE RESERVOIR REACH OF THE SANDY RIVER FOLLOWING REMOVAL OF OREGON'S MARMOT DAM

Following the 19 October 2007 removal of Oregon’s 14-m-tall, 50-m-wide Marmot Dam, the Sandy River rapidly incised and widened a channel through the ~730,000 m³ of impounded reservoir sediment. Initial sediment erosion and channel development was documented with time-lapse photography and traditional channel surveys, which allowed geometry, processes, and rates of channel development to be constrained. Within 1 year after removal, the reservoir reach achieved a state of quasi-equilibrium, but how closely and how quickly it approached pre-dam conditions remained uncertain.

Comparison of pre-dam and post-dam-removal spatial data was used to assess how close post-removal reservoir geometry is to the pre-dam state. Attempts to establish pre-dam conditions in the reservoir reach, a high-gradient (0.0056 m/m) active channel (30 to 70 m wide) were made by digitizing and georeferencing a 1911 topographic map provided by Portland General Electric. The map extends ~1 km upstream of the dam site, covering approximately one-third of the reservoir reach. Channel gradients following dam removal, based on linear fits to a time series of surveyed longitudinal water-surface profiles through the reservoir and channel immediately downstream, range from 0.01 on 5 November 2007 to 0.0065 on 21 January 2009, approaching a pre-dam gradient of 0.0056. Concurrently, channel widths in the reservoir increased. One site 500 m above the dam shows an increase of 59 m to a total width of almost 70 m, approaching that of its pre-dam form.

Rates of change within the reservoir reach slowed within the months following dam removal, and now large events (up to 5-yr recurrence interval flows) produce relatively little morphologic change. Present channel stability within the reservoir reach will likely persist until flows sufficiently large to overcome channel bed resistance occur. Continued examination of time-lapse photography and topographic surveys along with investigations of grain-size distribution will refine rates of reservoir response and provide a clearer understanding of how changing bed-material grain size influenced rates of channel stability.