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Co) C'') ...... co .- I-~' dam became colder and increased in volume as the reservoir filled. The larger volume of colder water did not warm as quickly as the water warmed during the first few years after the dam was completed, and it was still cold when it joined with the Yampa River. This change in volume, and the resultant decrease in temperature, may be a major reason why larval Colorado pikeminnow now drift so far downstream in the upper Green River. In addition to a temperature gradient, other factors may affect larval drift distance. A swifter river would be expected to push larvae further downstream than a slower river, and a river with little complexity (bends and current shifts) could keep larvae from reaching shoreline nursery habitats longer than a more-complex channel. Based on Green River information, it would appear that distance is not a requirement, but habitat accessibility may be more important. In 1987 and 1988, Platauia (1990) collected 7 of!3 YOY Colorado pikeminnow from RM 24 to RM 123 of the San Juan River, and five of the other six Colorado pikeminnow were collected in the lower 20 miles of the rivernear the Lake Powell interface. Of the 22 YOY collected during the 7-year research period, none were found above the lower 25 miles of the river. This indicates that most YOY Colorado pikeminnow in the San Juan River drift over 100 miles from the spawning area at RM 131-132 (although other spawning areas may not have been found), but not all did. Nursery habitats were available well above the lower 25 miles of the river (BIiesner and Lamarra 2000). It is not known why larval Colorado pikeminnow appeared to drift by favorable habitat during the 7-year research period, or why many found habitat in the middle river in the late 1980s. It is possible that sufficient larvae were not produced during the 7-year research period to adequately test ifriverIength below the spawning area is a limiting factor for Colorado pikeminnow. Two large juvenile Colorado pikeminnow were captured in the lower 12.9 miles of the San Juan River in 1996 (Ryden 2000a). Capture of these fish suggested that recruitment from fish that likely grew up in Lake Powell, or the lower section of the river, still occurred. This raises the question of whether Lake Powell, or the nursery habitat in the lower river, is actually a detriment to Colorado pikeminnow success. AIl drifting larvae found in the lower river may not necessarily be eaten by nonnative predators. Water Temperature As Navajo Reservoir filled, water released from the dam turned colder. Bliesner and Lamarra (2000) showed that sununer temperatures dramatically declined in the river from the dam downstream to the mouth of the Animas River. Figure 3.17 shows the change at Archuleta, New Mexico, about 7 miles below the dam, for pre- (1950-1961) and post-dam (1964-1968) periods; mid-sununer water temperatures declined about 5" C. Nonnative trout replaced native fishes as the dominant species for about 10 to 20 miles below the dam (RM 205). Flannelmouth sucker, bluehead sucker, and speckled dace, species that inhabit and reproduce in both cooler tributaries and warmer mainstreams in the Colorado Basin, were stilI very abundant from RM 205 to the confluence with the Animas River (RM 180.0). It is not known if Colorado pikeminnow and razorback sucker would use the area above the Animas River confluence. In a similar situation in the Green River below Flaming Gorge Dam, flannelmouth sucker, bluehead sucker, speckled dace, September 2000 3-39 Program Evaluation Report