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(1991 a) speculated that there have been no major changes in population size since the 1970s. Because strong year classes are infrequent, recruitment may not support the present numbers over a long period of time (Osmundson and Burnham 1996). Most of ~ the adult fish are found in the upper 60 mi of river below the Grand Valley diversion (Valdez et al. 1982, Osmundson et al. 1995, 1997). Adults also occur in the lower Gunnison River (Burdick 1995), and spawning was confirmed by capture of larvae in 1994, 1995, and 1996 (Anderson, unpublished data). The younger fish tend to occupy lower river reaches (Valdez et al. 1982, Osmundson et al. 1997). ~ The San Juan River contains a small population of Colorado pikeminnow, and collections of young fish indicate that the population is reproducing (Platania et al. 1991, Ryden and Ahlm 1996). In general, each of the known populations is associated with a primary nursery area ~ that harbors larvae from one or more spawning sites. In the Colorado River, the Green River system, and the San Juan River, adults are most prevalent in river reaches at, or upstream of, spawning areas (Tyus 1986, Tyus and Haines 1991, Osmundson and Burnham 1996, Ryden and Ahlm 1996). However, the present concentrations of adults may be restricted to river reaches below barriers that block migrations. Examples of ~ such blockages include Flaming Gorge Dam on the Green River (Vanicek 1967) and Taylor Draw Dam on the White River (Trammell et al. 1993). Humpback chub ~ Information about the historical distribution of humpback chub is sparse, in part, because the fish was not described as a species until relatively recently (Miller 1946, Valdez and Clemmer 1982). The largest extant population of humpback chub occurs in the Grand Canyon in the vicinity of the confluence of Little Colorado River (LCR) and Colorado River. Recent estimates of the population place the number of adults in the range of 4,500 to 10,400 fish (Douglas and Marsh 1996), which includes about 2,680 to 4,280 in the mainstream (Valdez and Ryel 1995). The humpback chub was first reported in the UCRB in the 1970s (Holden 1977, Valdez and Clemmer 1982), however museum collections document the existence of a population in the Yampa River in 1948 (Tyus 1998). The humpback chub has persisted in the Colorado and Green river systems, and is reproducing successfully in the Yampa and upper Colorado rivers ~ (Tyus et al. 1982, Archer et al. 1985, Kaeding et al. 1990, Karp and Tyus 1990). In the upper Colorado River, Valdez et al. (1982) captured 238 humpback chub, but nearly all (229) came from Black Rocks and Westwater Canyon. The population in Westwater Canyon probably consists of several thousand fish, but the precision of the estimate is very poor (B. Burdick, personal communication; T. Chart, unpublished data). Population size of humpback chub in Black Rocks and Westwater canyons is thought to be relatively stable (Kaeding et al. 1990, McAda et al. 1994).