GEOMORPHIC CHANGES RESULTING FROM RIVER MILL DAM OPERATIONS, CLACKAMAS RIVER OREGON

River Mill Dam on the Clackamas River was completed in 1911 to provide hydroelectric power to the burgeoning city of Portland, Oregon. It is one of the oldest dams of its size in the western United States. Hydrologic changes from the dam are minimal, but a set of dam-induced geomorphic changes resulting from sediment supply changes has been documented. Incision immediately below the dam is minor due to erosion-resistant bedrock. Regularly spaced bedrock pools up to 10 m deep have been eroded into bedrock for 3 km below the dam. Average pool spacing is 250 m or approximately 3.6 channel widths. Deep pool formation may be related to sediment supply reduction and channel degradation below the dam. Measurable geomorphic changes below the dam also include: 1) increase in surface grain-size; 2) reduction in side channel area; 3) stripping of bar alluvium; 4) exposure of bedrock; and 5) lowering of water surface elevations. Sediment input sources for 13 km below the dam are limited to mass wasting and lateral erosion of Holocene terraces and islands. For about 3 km below the dam, there is approximately 300,000 m3 of sediment stored in islands, bars, and in the channel. This represents 13 years of bed load transport based on reservoir trap data for River Mill Dam. Between 1908 and 2000, water surface elevations of the Clackamas River dropped an average of 0.8 m for 17 km below River Mill Dam; remained unchanged for the next 6.4 km; rose 1 m for the next 11.3 km; and dropped 1.2 m for the last 3 km. Surface grain D50 is 2 to 3 times higher for approximately 2 to 3 km below the dam.