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1 1 The Groundwater Seepage Hypothesis, proposed for other species by Harden-Jones (1981), has been advanced as a possible homing mechanism for Colorado squawfish (Tyus 1985). Although no experimental evidence to date confirms or disproves the existence of an olfactory imprinting mechanism for Colorado squawfish, observations at the two confirmed spawning grounds in upper Green River basin indicate that these canyon areas are geohydrologically unique. Colorado squawfish may be oriAnting to these areas because of freshwater input from spring-fed tributaries and sandstone/limestone seeps (e.g., Florence Creek in Gray Canyon and Warm Springs Creek in Yampa Canyon). Spawning requirements of Colorado squawfish were evaluated at the Yampa 1 River spawning reach, 1981-1987. Both water temperature and discharge was highly variable during the optimum spawning period (Table 2). Water temperatures ranged from 14.5-27.5C, average minimum and average maximum temperatures for all years were about 19C and 24C, respectively. Mean discharge ranged from 893 cfs in 1981, to 3,825 cfs in 1982. There was no apparent relationship for spawning flows between years, except that spawning always occurred with declining water levels following peak spring floods (Figure 7). Although we did not correJ_ate discharge with spawning period, the mean discharge for 1981-1987 (2,436 cfs) was similar to that reported by Butler (1988) for years 1941-1986 (2,140 cfs). Mean discharge during the optimal spawning period was similar for the four high water years (1983-1986), but there was no agreement between the two low flow years (1981 and 1987). These inconsistencies emphasize the need for long-term studies in order to investigate relationships between physical factors and biological phenomena. As indicated by Figure 8, there is general agreement between period of migration, collections of ripe fish, and estimated dates of egg deposit, back- 15