LIDAR ASSESSMENT OF SEDIMENT TRANSPORT RELATED TO THE REMOVAL OF MARMOT DAM, SANDY RIVER, OREGON

In October 2007, Marmot Dam on the Sandy River, Oregon, was removed, releasing an estimated 430,000 cubic meters of sediment downstream. Field observations in 2007 – 2010 following the dam removal documented deposition of nearly half of the eroded sediment (215,000 m³) in the first 2 km downstream of the dam but the fate of more than 200,000 cubic meters of chiefly sand transported farther downstream is uncertain. In the current study, four sequential LiDAR data sets from 2006 – 2010 were used to quantify sediment storage and erosion in the 40 km from the dam site to the confluence with the Columbia River in an attempt to document the unaccounted sediment eroded from the reservoir. We hypothesized that a pulse of sediment from the dam removal would be distinguished by a successive downstream growth of sediment bars through time.

The LiDAR imagery includes two data sets acquired before the dam removal and two afterward. Esri ArcMap 10 was used to process the LiDAR-derived DEMs to determine changes in surface elevation between successive images. The data resolution allowed for creation of 3-meter DEMs. Differencing two DEMs resulted in a new raster showing the locations of erosion and deposition for that period of time. A gain in elevation represents deposition of sediment while a loss represents erosion. The locations, vertical elevations, and horizontal dimensions of sediment bars were measured for each year to calculate the change in the volume of stored. Each DEM was differenced from the previous year in order to show the sequential change in sediment distribution. The net erosion and net deposition were determined for each successive pair of LiDAR acquisitions to determine the cumulative volume of mobilized sediment and the spatial distribution of erosion and deposition. These observations will be used to determine whether the previously unaccounted sediment can be located, as well as any systematic patterns in sediment redistribution over the 4-year period.