ROLE OF SLOPE STABILITY IN CUMULATIVE IMPACT ASSESSMENT OF HYDROPOWERDEVELOPMENT - NORTH-CASCADES, WASHINGTON

Two environmental assessments considered the potential cumulative envi ronmental impacts resulting from the development of eight proposed hyd ropower projects in the Nooksack River Basin and 11 proposed projects in the Skagit River Basin, North Cascades, Washington, respectively. W hile not identified as a target resource, slope stability and the alte ration of sediment supply to creeks and river mainstems significantly affect other resources. The slope stability assessment emphasized the potential for cumulative impacts under disturbed conditions (e.g., roa d construction and timber harvesting) and a landslide-induced pipeline rupture scenario. In the case of small-scale slides, the sluicing act ion of ruptured pipeline water on the fresh landslide scarp was found to be capable of eroding significantly more material than the original landslide. For large-scale landslides, sluiced material was found to be a small increment of the original landslide. These results predicte d that hypothetical accidental pipeline rupture by small-scale landsli des may result in potential cumulative impacts for 12 of the 19 projec ts with pending license applications in both river basins.