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McAda and Wydowski 1980), we speculate that this population may be increasing in number, perhaps due to favorable runoff conditions in the Yampa watershed in the mid-1980s. However, the current paucity of Colorado River chubs in Split Mountain and Whirlpool canyons of the upper Green River, relative to earlier decades (e.g., Vanicek et al. 1970; Holden and Stalnaker 1975a) indicates a general decline of Gila in that system. This trend may be related to stabilization of flows and the loss of natural seasonal and annual flooding due to regulated flow releases from Flaming Gorge Dam. Although spawning was not observed due to high turbidity, humpback chub spawned in Yampa Canyon as indicated by capture of ripe males and females in the same shoreline eddies, and capture of young fish (88-228 mm TL). However, spawning of humpback chub in the DNM portion of the Green River (i.e., Whirlpool Canyon) was not indicated. A single ripe male was collected there in 1988, but it is unknown whether that fish was attempting to spawn in the Green River, or was a stray from the Yampa River. Ripe humpback chub were collected during the period of declining spring flows and increasing river temperatures that followed peak runoff. This occurred in May and June in low (e.g., 1987, 1989) and average (e.g., 1988) flow years, but the spawning period extended into July in the high flow year 1986. Sampling during pre-runoff and late post-runoff yielded no humpback chub in breeding condition, and we presume the fish spawned only during the 4-5 week period following highest spring flows. Ripe fish were captured in Yampa Canyon at temperatures (200C) that approximated optimum egg incubation conditions (Marsh 1985). River 13