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- . .....: .. -- I 002 H 3 The seasonal hydrograph of the Yampa River has not been substantially modified by large dams and reservoirs or large out-of-basin diversions, The Yampa River is the only stream of its size in the Upper Colorado River Basin in which spring peak flows have changed relatively little since water development began near the turn of the 20th Century. Spong peaks result from melting snowpack accumulated at higher elevations during the winter. Spring runoff typically begins as early as mid- March and wanes no later than mid-July, with peak flows at Maybell occurring between April 25 and June 19 (Figure 2). However, more than 60% (57 of94 occurrences) of these occurred within a 3-week period (May 10-31), during which period more than one-fourth of the average annual discharge passed the Maybell gage. A typical spring snowmelt hydrograph consists of three distinct segments: (I) ascending limb, (2) peak, and (3) descending limb. Each of these segments serves a specific function to maintain the aquatic habitats essential for the endangered fishes, initiate pre-spawning and post-spawning 'migrations, cue spawning behavior, and transport larval fish to nursery habitats downstream. However, snowmelt at lower elevations can produce early minor peaks prior to onset of the major peak, and severe thunderstorms occasionally produce transient peaks in the hydro graph during the summer. From 1916 through 1998, the highest flow recorded at the Maybell gage was 24,400 cfs which occurred on May 17, 1984, while the lowest peak flow (3,180 cfs) was recorded on June 5 and June 10, 1977. Flow maxima from 6,000 to 12,000 cfs occurred at Maybell in 56 out of83 years (67%) with an average recurrence interval of 1.5 years (Figure 3). Peak flows greater than 12,000 cfs occurred in 9 of 83 years (I I %), while peaks less than 6,000 cfs occurred in 18 of 83 years (22%). Flows as high as 32,000 cfs have been recorded by the gage at DeerIodge Park, 5 RM downstream from the confluence of the Little Snake River, the largest tributary to the Yampa. Peak flows are particularly important for transporting sediment that creates and maintains suitable spawning habitats for the endangered fishes in the Yampa River, as well as numerous back-waters and floodplain depressions along the Green River from Jensen to Ouray, Utah. These floodplain habitats serve as nurseries for the Colorado pikeminnow and razorback sucker, critical to survival oflarvae (Andrews 1978, 1986; Elliott et al. 1984; O'Brien 1987). These relatively quiet, warm, shallow habitats allow smaller fish to escape predation and grow more rapidly than in the more rigorous environment of the main channel. Rapid growth is the key to their survival and eventual recruitment into adult populations. Recovery goals for the Colorado pikeminnow and razorback sucker (USFWS 2002c,d) require establishment and maintenance of self-sustaining populations of these species in the Green River. Adequate, suitable spawning and nursery habitats are essential to recovery of Yampa/Green River populations of these fish species (Day & Crosby 1997; Holden 1978,1980; Muth et al. 2000; Rakowski & Schmidt 1996; Schmidt 1996; Tyus 1987; Tyus & Karp 1991; Wick 1997). Therefore, the peak flows that provide these habitats are critical to recovery, The Yampa River not only contributes as much water as the Green River, but also provides a more natural shape to the hydrograph downstream from their confluence (Figure 4). The hydrographs in Figure 4 were derived by averaging daily flow data across the concurrent period of record for the three gages (1982-1994). They show that the hydro graph at Jensen, on average, approximates the sum of the hydrographs at Greendale and Deerlodge. They also demonstrate the differences between the highly regulated Green River hydrograph (Greendale) and a relatively unregulated Yampa River hydrograph (Deerlodge). ~ " ,. Management Plan for Endangered Fishes in the Yampa River Basin 5