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N t WYOMING SCALE IN KILOMETERS o 50 100 150 200 250 300 350 .00 so 100 150 200 250 SC.AL.E IN Mn..ES ......." ......,...... I \, ; r--- r-,...---- -...----1 .....- '\ -- MEXICO ---. \ \, Figure 2.--Historic (light stipple) and existing (dark stipple) range of the Colorado squawfish in North America. Holden (1973), Seethaler (1978) and others. Squawfish were distributed erratically throughout the basin, including the mid-lower reaches of the mainstem in Utah and Colorado, and its principal tributaries, the White, Gunnison, Green and Yampa Rivers. The species' range as detailed by the above investigators was, with some resolution, fundamentally correct as revealed by subsequent field studies initiated by the Colorado Division of Wildlife (DOW) in 1977 and the Colorado River Fisheries Project (CRFP) of the U.S. Fish and Wildlife Service in 1979. Critical life history data were limited to observations of habitats occupied by adults and timing of sexual maturity. Absolutely no observations of natural spawning had been documented. The early life history stages of P. lucius were known as a result of hatchery propagation at Willow Beach National Fish Hatchery, Arizona, by Toney (1974) and Hamman (1981) and larvae/ juveniles were available to researchers for comparison with field specimens. The consensus of opinion prior to DOW/CRFP studies was that if the Colorado squawfish spawned at all in Colorado, it would be in the lower mainstem. The Yampa River was believed to be very near the edge of the species' range and far too small to support spawning for such a large fish. THE LOWER YAMPA RIVER ECOSYSTEM The Yampa River (fig. 2) is the principal tribu- tary of the Green River sub-basin. It heads in the Park and Gore ranges and the Flattop Mountains in northcentral Colorado and flows north to Steamboat Springs (Routt County) where it turns west to the Green River confluence in Dinosaur National Monument (Moffat County). The river is approximately 200 mi in length, and the last 47 mi are wholly within the boundaries of the national monument. The Yampa exhibits wide annual fluctuations in physicochemical charac- teristics, due largely to its snowpack dependence and the semi-arid nature of the watershed over a great Pjrt of its course. Flows can range from <100 ft /sec during the base flo~ period of September-February to >20,000 ft /sec during the last week of May when peak flows are typically recorded. Water temperatures similarly fluctuate widely, i.e., from 1.0 C (winter) to approxi- mately 30 C (July and August). Turbidity (nap- thalmic turbidity units) ranges from <1.0 during fall-winter base flows to >100 during peak runoff and following prolonged storms (USGS 1983; O'Brien 1984). Color is brown to brick-red during runoff and often through August. The Yampa River in Dinosaur National Monument has a mejn annual flow of approximately 50 billion ft (National Park Service 1979) and it is the last large free-flowing river on Colorado's western slope. In contrast with the mainstem Colorado River and its other principal tribu- taries, present Yampa flows appear to approximate historical discharge patterns with normal sea- sonal fluctuations. The lowermost 46 mi of the Yampa River are entirely contained within deep canyons (Yampa Canyon). Because of the Canyon's geology, the river may be subdivided into an upper, high- gradient reach (river mi 46-20) characterized by a boulder-cobble substrate and a lower, low- gradient reach (RM 20-0) where sands and silts increasingly replace the large boulders. Riverine habitat varies from major/minor rapids and eddies, particular.ly in the upper reach, to relatively deep, quiet pools and braided sections interspersed with cobble bars in the lower reach (O'Brien 1984). As is the case with essentially all reaches of the Colorado River system, several species of exotic fishes have become established in the Yampa River (Haynes and Muth 1984) and introduced tamerisk (Tamarix gallica) is becoming estab- lished on the beaches. However, unlike the dam- controlled Green River with which it joins, relatively natural seasonal Yampa flows have maintained a largely native riparian ecosystem with deposition of driftwood and local cottonwood (Populus fremonti) germination during wet years. Field studies in Yampa Canyon are logistically difficult, requiring travel by inflatable raft and canoes and fish sampling is inefficient, at best. Studies, with the expressed intent of determining the likelihood of squawfish spawning in the lower Yampa River, were initiated in 190