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442E . . . In California suckers often occur in the finest trout streams, but are not considered a problem (Burns, 1966). The western longnose sucker has become competitive with trout in some reservoirs in Colorado, including Turquoise Lake. On the other hand, the suckers and trout coexist in Twin Lakes without apparent effects on the game fish. In general, problems involving suckers vary with the lakes and streams where they exist, and ~ust be identified and solved from case to case (Lagler, 1956). The Colorado Division of Wildlife has been attempting to establish opossum shrimp (Mysis relicta) in reservoirs in Colorado. This species is restricted to deep cold waters and cannot survive in streams at all. The shrimp have become an important food source for trout in up to 45 lakes and reservoirs in Colorado, Utah, and Wyoming, during the late 1960's and early 1970's. South Dakota has recently been receiving mysis transplants. Dams, including diversion dams and structures, create a barrier to the upstream migration of fish. Since many fish species migrate upstream to spawn, the total spawning area available is effectively reduced by dams. There is a total of 20 additional dams that are planned for the full Project. The dams will also regulate the downstream flows. The stabilization of flows will eliminate the scouring effect of the spring runoff during dry years. During wet years, spilling over the diversion dams will occur and, conse- quently, the scour as well. Without the scouring, sediment tends to accumulate in gravel beds. In order for these to be utilized for spawning beds, they must be clean and well aerated. Sedimentation on the beds following spawning can suffocate the embryos as well. Sedimentation to varying degrees can occur during construction. This can also eliminate or reduce the benthic community. However, when Ruedi Dam was constructed, sedimentation basins were con- structed below the construction site. In the process, gravel beds were exposed that have been instrumental in increasing the spawning success of the resident brown trout population. The downstream reach has been experi- mentally maintained through natural reproduction since Ruedi became operational. Downstream migrations will cause an intermingling of aquatic organisms and fish due to interbasin and inter- drainage transfers of water, and cause the establishment of subpopulations at stream headwaters, in the impound- ments, diversion pools, and below impoundment structures. IV-36