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11/14/01 draft report, Schmidt and Box development, the Colorado pikeminnow was distributed between Wyoming and the lower Colorado River near Mexico. The Colorado River is now fragmented by long reservoirs and high dams. The largest known extant populations of pikeminnows occur in the middle Green River. There are two major spawning sites for these populations: one in the lower Yampa River and the other in Gray Canyon, downstream from the Uinta Basin (Tyus and Haines 1991). Flaming Gorge Dam is located on the Green River, 105 km upstream from the confluence of the Green and Yampa Rivers (Fig. 1). This paper concerns the distribution of age-O fish produced by fish that spawn in the Yampa River. Life History of the Colorado Pikeminnow and Characteristics of Backwaters Spawning on Yampa River gravel bars is triggered by increasing water temperature and occurs a few weeks after the flood peak, usually in late June or early July (Bestgen et al. 1998). Embryos hatch a few days after spawning, develop in the substrate, emerge, and drift passively downstream to the Green River (Tyus 199L)3estgen and Williams 1994). Larvae are approximately 6 to 7 days old and about8 or 9~m in length when they are enter the Green River '--. .~ (Bestgen et al. 1998). ~~ t.- ' Larvae may drift hundreds of kilometers (Bestgen et al 1998; Dudley and Platania 2000a), and there is a several-week period when the limited swimming ability oflarvae restricts them to exogenous feeding in backwaters where velocity is very low. Little is known about the mechanics of downstream drift, although the density of larval drift is measured at the confluence of theYampa and Green Rivers. Dudley and Platania (2000a) determined that Colorado pikeminnow larvae in the San Juan River drift with the same speed as neutrally-buoyant particles, approximately 80 percent of the mean stream speed (S. P. Platania, University of New Mexico, oral commun., 2001). Bestgen et al. (1998) estimated that the fastest drift rates approximate the mean stream velocity and deliver larvae to the downstream part of our study area a few days after they enter the GreenRiver. He estimated that the slowest drift rate, based on the minimum velocity of flow through near-shore . sampling nets, is about 0.5 m/sec. The primary nursery areas for age-O fish are backwaters in the Green River (Tyus and H~nes 1991). Little is known about transport of larvae into backwaters or their retention there. The rate at which larvae enter and leave backwaters depends on physical and biological factors. Larvae gain horizontal mobility as their body size increases; their swim bladders inflate, and they begin exogenous feeding. In the absence of swimming ability, the rate of movement of larvae into and out of backwaters must be due to hydraulic factors. As larvae grow and gain swimming ability, they may also enter or leave backwaters intentionally (Paulin et al. 1989; Bestgen et al. 1998). It is unknown if they exhibit schooling behavior at this age. The growth of larvae and onset of the physiological changes that increase swimming ability are related to water temperature (Childs and Clarkson 1996). 5 ~