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.' . . Area Manager DRAFT 40 Lake Powell is populated by several fish species not native to the Colorado River that are predators on native fish. As mentioned earlier, larval native fish that drift into Lake Powell are aimOSI certainiy iost to predaiiun by laIgtanuuth bass, smalL-nout; bCi3S, y...alleye, or crappie (Pomoxis sp.). Among the species present is striped bass (Marone saxatilis), which migrates up the San Juan River to spawn. It has been found as far upstream as the PNM weir (RM 166) (Davis 2003). Adult striped bass are piscivorous (Moyle 1976). In 2000, 432 striped bass were captured during monitoring trips for pikeminnow and during trips to remove non-native fishes (Davis 2003). The contents of 38 stomachs were analyzed and native suckers were found in 41 percent (Davis 2003). This migratory predator is a threat to larval and juvenile native fish. Changes in the timing and magnitude of flows Typical of rivers in the Southwest, the San Juan was originally characterized by large spring snowmelt peak flows, low summer and winter base flows, and high-magnitude, short-duration summer and fall storm events (Holden 1999) (Figure 2). Historically, flows in the San Juan River were highly variable and ranged from a low of 44 cfs in September 1956, to a high of 19,790 cfs in May 194J (mean monthly values) at the U.S. Geological Survey (USGS) Station gauge near Shiprock, New Mexico. The lowest mean monthly flow at Shiprock after Navajo Dam was filled was 213 cfs (August 1978) and the highest mean monthly flow was 9,508 (June 1979), a decrease of over] 0,000 cfs compared to pre-dam years. For the 40 years of record post- dam, the highest spring peak flow (daily mean) recorded at the USGS gauge at Bluff, Utah, was 15,200 cfs (May 30, 1979). For the 49 years of record prior to Navajo Dam a peak spring flow greater than 15,200 cfs occurred 13 times (25 percent of the time). The highest spring peak flow recorded (daily mean) was 52,000 cfs (June 30, 1927). Flows from the Animas River (the largest tributary downstream of Navajo Dam) are rarely above 10,000 cfs. Since 1914, the highest daily mean peak flow recorded was] 1,000 cfs (June] 9, ] 949) and spring flows over 10,000 cfs occurred in only 2 other years. The maximum release from Navajo Dam is 5,000 cfs. Although there are additional tributaries downstream that contribute water to the San Juan, it is clear that peak flows will never reach the magnitude that they once did. The completion of Navajo Dam and subsequent dam operations altered the natural hydro graph of the San Juan River substantially (Holden 1999). There was an appreciable reduction in the magnitude and change in timing of the annual spring peak. In wet years, dam releases began early to create space in the reservoir to store runoff (Holden ] 999). The peak discharge averaged 54 percent of the spring peak of pre-dam years. Base flows were substantially elevated in comparison to pre-dam years. The median monthly flow for the base flow months (August- February) averaged 168 percent of the pre-dam period (Holden 1999). Minimum flows were elevated and periods of near-zero flow were eliminated with a minimum monthly flow during base-flow periods of250 cfs compared to 65 cfs for the pre-dam period (Holden 1999). The hydrograph became much flatter during this period oftime (Figure 2). From 1992 to 1997 (research period) and continuing today, a more natural hydrograph has been maintained but as described in the previous paragraph, peak magnitudes cannot be matched because of outlet restrictions at the dam (Figure 2). In addition the total amount of water available to the river is reduced due to depletions (discussed below). OOlH2