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_ , P"' ,,,~ .,. OUl~ti3 Land and Facility Resources landslide mass. Both conditions would contribute to instability of the landslide mass and may trigger movement that would be detrimental to the railroad. Erosion of the toe of the landslide mass due to increased flow velocities of the Colorado River would contribute directly to landslide instability. The removal of material by this erosion process essentially removes weight that helps stabilize the landslide mass. Therefore, any erosive action at the toe of the landslide is undesirable. mcreased flow velocities would be acceptable if down cutting or scouring did not occur near the landslide. A rise of the water table within the landslide mass would also contribute to landslide instability. As water levels rise within a landslide mass, pore-water pressures are increased and slippage along a water-saturated slip plane is more likely to occur. Furthermore, a sudden increase or decrease in the water table may trigger movement. A gradual decline and maintenance of a lower overall water table would increase the stability of the landslide. The possibility of future movement is high since the area is very unstable and natural climatological and/or hydrological conditions could easily trigger movement of this slide. Impacts No Action: The Jacobson Hydro No. I Project proposes to raise the water level with flashboards on the darn, and the 1990 FERC license requires development of an erosion control plan for review by the railroad. The fixed flashboards would raise the water table by approximately 4 feet. This could cause a slight decrease in landslide stability. Fish Ladder (with Hydropower Plant): Construction of a fish ladder around the existing diversion darn would have little or no effect on the stability of Tunnel No.3 Landslide provided there is not an overall increase in the river water surface elevation. As for No Action, the Jacobson Hydro No.1 Project proposes to raise the water level with flashboards on the dam, but the flashboards would be adjustable. Any sudden lowering of the flashboards would result in a rapid drawdown of the water surface elevation, which could contribute to instability of the slide. Fish Ladder (without Hydropower Plant): Construction of a fish ladder around the existing diversion darn would have little or no effect on the stability of Tunnel No.3 Landslide provided there is not an overall increase in the river water surface elevation. Dam Removal: Removal of the Price-Stubb Diversion Dam would change river dynamics upstream of the darn in the vicinity of the Tunnel No.3 Landslide. A preliminary scour study conducted by Reclamation's Technical Service Center (Lyons, 1998) shows the average flow velocity of the river would increase in the reach from the diversion dam upstream to the Colorado River Siphon. However, this study indicated no channel degradation would be anticipated since there is no extensive area of sediment deposition upstream of the dam. 33