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C) C) ..... co <:D RANGE LIMITATIONS Introduction Range limitation is the restriction of the area of habitat available to a species, and in rivers it typical]y occurs as a result of a major change in habitat length, such as a reservoir inundating a portion of a river, a blockage of access to a portion of river (e.g., by a dam), or a change in water quality that makes a portion of the river unusable by certain fish species. When the SJRIP was initiated, it was known that range limitation was a potential limiting factor to endangered fish species recovery within the river. Range limitation may reduce the possible overall population size of the endangered species by limiting available habitat, or by the loss of a portion of the range that included a key habitat. In the Green and Colorado rivers, Colorado pikeminnow larvae drift from 100 to 200 miles below spawning areas to find nursery habitats. Drifting that far in the San Juan River would result in the larvae entering Lake Powell, a poor nursery habitat because of abundant nonnative predators. A]though no studies directly addressed range limitation, data collected by several studies allowed analysis of this issue. .I::Iabitat Length. The length of riverine habitat for the native fishes in the Sanjuan River was dramatically reduced by Lake Powell and Navajo Reservoir. Lake Powell inundated the lower 54 miles of the river (B1iesner 1999a), and Navajo Reservoir inundated another 27 miles. The two endangered fishes undoubtedly utilized the lower 54 miles of the river, and Colorado pikeminnowused 27 miles of what is now Navajo Reservoir for habitat, and roundtail chub used the entire inundated area. It is not known if any key habitats existed in the inundated areas but, based on collections of fish in that area and the location of other known spawning areas in similar upstream portions of other rivers (e.g., Yampa River), a Colorado pikeminnow spawning site was possible within the Navajo Reservoir section. It is probable that the lower San Juan River provided nursery habitat for Colorado pikeminnow much as it does below RM 13].0 today. Compared with the study area, the river in the Lake Powell reach was of similar, relatively steep gradient and did not provide the type oflow-gradient reaches found in the Green and Colorado rivers (Figure 3.16) that are usually associated with Colorado pikeminnow nursery areas. The river in this reach may have been similar to the canyon reach (RM 20 to RM 65) of the SJRlP study area where backwaters and other low-ve]ocity habitats are rare. Low-gradient, flooded bottomland habitat also did not ]ikely occur in the Lake Powell reach; hence habitat for young razorback sucker would ]ikely have been similar to the present study area. The two reservoirs reduced potential range and habitat for the two endangered fishes from about 325 miles to only 225 miles and inundated potential Colorado pikeminnow spawning areas in the upper San Juan River. Some Colorado pikeminnow in the Green and Colorado river systems drifted up to 200 miles from spawning areas before finding nursery habitat, although some used nursery areas only a few miles below the spawning areas (Trammel and Chart] 999, Trammell et al. ] 999). Questions not completely answered are: do the larvae need to drift so far, is the length of the drift related to the accessibility and availability of nurse!)' habitat, or is the length of the drift related to some other factors? From ]964 to 1966, Vanicek and Kramer (1969) found relatively large numbers ofYOY, age-I, and age-2 Colorado pikeminnow in the Green River from the mouth of Yampa River to September 2000 3-37 Program Evaluation Report