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uOlGlJ9 Revised Supplemental Draft Environmental Assessment-Chapter 3-Affected Environment and Environmental Consequences In February and March 1988, movement of the landslide occurred again. No damage was done to Reclamation facilities, but railroad traffic was disrupted as the tracks had to continually be realigned. To halt the movement of the landslide, the Denver and Rio Grande Western Railroad, owners of the railroad then, removed material from the top one-third of the slide and stockpiled it just downstream of the slide. No evidence of further movement has been observed or reponed since this material was removed. It is not know what triggered movement of this slide in 1950 and 1988. No clear correlation is evident with high precipitation events. However, the entire area is over- steepened and in a state of delicate balance. Long-term changes in moisture content within the slide mass, or removal of supportive material at the toe may have contributed to the historic movement. The stability of this landslide becomes an issue if the proposed fish passage significantly alters river dynamics. Two basic concerns are: 1) potential erosion of the toe of the landslide caused by increased flow velocities in the river, and 2) potential rise of the water table within the 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. Increased 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 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 terminated Jacobson Hydro No. 1 Project proposed to raise the water level with flashboards on the dam, and the 1990 FERC license required development of an erosion control plan for review by the railroad. The fixed flashboards would raise the water tabl,e by approximately 4 feet. This could cause a slight decrease in landslide stability. Without the terminated Jacobson Hydro No.1 Project, the No Action alternative would have no affect on the Tunnel No.3 landslide. 40