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460 Schmidt and Box Table 1. Hydrology of the Upper Green, Yampa, and Middle Green Rivers Upper Green River (09234500, Greendale, UT) Yampa River (09260050, Deerlodge Park, CO) Middle Green River, Yampa River to White River (09261000, Jensen, UT) Green River, downstream from White River (09315000, Green River, UT5) 60(1923-1996) 1 Mean annual flow (m' s -1) 2-yr recurrence flood (m3 s - 1) Mean annual sediment load (Mg) I Grams and Schmidt (2002) 2 Allred and Schmidt (1999) 'Thompson (1984) 4 Andrews (1986) 51ncludes stream flow from the Price River and ungaged tributaries downstream from the study area. Pre-dam Post-dam Pre-dam Post-dam Pre-dam Post-dam 59 (1951_1962)1 58 (1963-199W 339 (1931-1962)1 147 (1963-1996)1 3,600,0004 04 399 (1927-1962) 1 400 (1963-199W 1,900,0004 1,900,0004 122 119 626 (1923-1962) 1 480 (1963-1996) 1 6,500,000 (1948-1962)1 2,800,000 (1963-197W Suspended-sediment transport in the middle Green River decreased 57 percent following completion of the dam (Grams and Schmidt 2002). The channel narrowed between 10 and 25 percent during the last century (Lyons, Pucherelli, and Clark 1992; Allred and Schmidt 1999; Grams and Schmidt 2002), and sediment budgets suggest that the narrowed channel is capable of trans- porting all of the fine sediment now delivered from tributaries (Andrews 1986). Overview of the Early Life History of Colorado Pikeminnow As with any endangered species, the small population hinders understanding many aspects of the Colorado pikeminnow's life history. All drifting larvae in the study area are derived from Yampa River spawning sites ap- proximately 30 km upstream from the Green River (Figure 1). Spawning begins in the last half of June, during recession of the Yampa River's spring flood, and lasts about one month (Bestgen, Muth, and Trammell 1998). Following emergence from the substrate, larvae drift downstream and arrive at the Green River approx- imately twelve days after spawning (Tyus 1991; Bestgen and Williams 1994; Bestgen, Muth, and Trammell 1998). The rate at which larvae enter the Green River varies greatly. During the month-long period of drift in the years of concern in this study, Bestgen, Muth, and Trammell (1998) estimated that larval abundance varied between 0 and 5,000 larvae/hr. Most of the drift in some years occun:ed on only a few days. In 1990, for example, 62 percent of all larvae were caught during five of the twenty nine days of drift. 156 (1930-1962)2 160 (1963-1996)2 740 (1930-1962)2 586 (1963-1996)2 19,500,000 (1930-63)3 8.700,000 (1964-1982)3 Larvae drift hundreds of kilometers downstream, but little is known about the mechanics or biology of this process. There is a several-week period when larvae have little swimming ability; available evidence indicates that larvae drift at the approximate speed of the main cur- rent. For example, discrete aggregations of larvae traveled 40 km in less than one day in 1990 (Bestgen, Muth, and Trammell 1998). Similar observations of drift speed have been made on the San Juan River where Dudley and Platania (2000a) simulated larval drift by adding neutrally buoyant beads. These beads traveled at approximately 80 percent of the mean stream speed. Larvae eventually are transported, or swim, into em- bayments of low, or zero, velocity adjacent to the main flow. Fish biologists call these habitats "backwaters," and they are the preferred nursery habitat for the remainder of age 0 (Tyus and Haines 1991). The greatest number of backwaters and the densest concentrations of age-O fish in the study area in late summer and fall are in the Uinta Basin where the channel is wider and the gradient is flatter than elsewhere (Figure 2). Backwaters are a wide range of sizes (Tyus and Haines 1991; Rakowski 1997; Day, Christopherson, and Crosby 1999), but age-O pike minnow seem to prefer large, warm, deep, and turbid backwaters that persist for the entire base flow period (Tyus and Haines 1991; Day, Christopherson, and Crosby 1999). Tyus and Haines (1991) found that backwaters that contained age-O pikeminnow were, on average, four times larger than unoccupied backwaters. The rate at which larvae enter and leave backwaters depends on physical and biological factors, but little is known about these processes. Larvae gain swimming ability as their body size increases, their swim bladders