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future water depletions is almost certain as the State of Colorado has proclaimed its intent to pursue the full utilization of its apportioned share of Colorado River water as set forth in the 1922 Colorado River Compact and the 1948 Upper Colorado River Compact, multi-state agreements formulated prior to societal concerns for environmental quality or the enactment of the Endangered Species Act. Against this backdrop, one can appreciate the conflict between water development and protection of endemic Colorado River fishes and the institutional tug-of- war over water flowing through the 15-mile reach. The Recovery Program intends to build fish ladders around the diversion structures upstream of the 15-mile reach so that Colorado squawfish and razorback suckers are allowed access to upstream reaches of river where they historically occurred. Although this extension of their present range might prove beneficial, particularly for the razorback sucker, the benefits to Colorado squawfish are probably small. From a biological standpoint, these upstream reaches cannot serve as a substitute for the 15-mile reach. Though Colorado squawfish and razorback sucker were historically found as far upstream as Rifle, Colorado (55 miles upstream of Palisade), this upper reach constituted the fringe or outer margin of their range; as such it was suboptimal habitat. Electrofishing surveys by the Fish and Wildlife Service and Colorado Division of Wildlife indicate that native suckers and chubs are abundant in the reach above Palisade and therefore forage for adult Colorado squawfish should be plentiful. However, the potential value of this reach to Colorado squawfish is probably low for the same reason that made it the historic fringe of this species' range, i.e., a marginal thermal regime. This stretch of river is the transition zone between the cold-water fishery upstream, dominated by salmonid species such as trout and mountain whitefish, and the warm-water fishery downstream of Palisade historically dominated by chubs, Colorado squawfish, and native suckers. Though temperatures in the Palisade-to-Rifle reach are only a few degrees centigrade lower than in the Grand Valley on any given day, this can mean the difference between meeting or not meeting threshold temperatures necessary for Colorado squawfish spawning. United States Geological Survey (USGS) gaging stations at Cameo (14 miles upstream of Palisade) and at the Colorado/Utah state line record water temperatures in addition to daily discharge. In 1994, FWS installed an additional thermograph at Rulison (45 miles upstream of Palisade) and in 1995 another one in the lower Grand Valley (21 miles downstream of Palisade). During the past four years, the upper Colorado River has experienced two relatively dry years (1992 and 1994) and two relatively wet years (1993 and 1995). Figs. 5 and 6 indicate that during low-water years, spawning temperatures (> 20 C) were attained at Cameo (1992) and at Rulison (1994); however, during wet years, temperatures either did not reach 20 C (Rulison 1995) or did so only briefly (Cameo 1993 and 1995). In years when spawning temperatures occur, attainment of 20 C is delayed approximately one month from the date when the same temperatures are met downstream at the state-line site. Thus, temperatures necessary for spawning are not always met in the Palisade-to-Rifle reach, and when they are, the duration of the first-year growing season for resulting larvae is relatively short. 7