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Open-File Report O-13-01, Change detection analysis using serial lidar data along a portion of the Upper Sandy River, Multnomah and Clackamas Counties, Oregon, by John T. English.
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The Sandy River is a dynamic river subject to significant geomorphic change. The combination of fine sediment, large woody debris, and frequent effective flows from rain-on-snow events allows the river to change its bed composition and location regularly. The floodplain contains numerous abandoned channels that readily recapture flow during large flows, requiring residents within the floodplain to pay close attention to the river's movement. Although large flows sometimes inundate the overbank areas, mass wasting caused by bank erosion and changes to bed formation are a major concern to residents; significant property loss has occurred in recent years. The removal of Marmot dam on October 17, 2007, changed the river's sediment regime as approximately 730,000 cubic meters of stored sand and gravel were released downstream. The Sandy River experienced two significant flow events greater than 25,000 cubic feet per second (cfs) since 2007 (based on peak flow results for Sandy River gauge 14142500). In January 2009 a large flow of 30,000 cfs occurred on the Sandy, causing significant channel change while destroying a several homes). In January 2011 another flood of 39,000 cfs occurred, wiping out several homes; again, the channel experienced significant erosion. The upper Sandy River continues to experience channel change as it cuts through the sediment of the former upstream reservoir. Scientists continue to monitor the Sandy River to better understand the post dam removal sediment and flow regime.