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because extended runoff during high water years delays warming of the river. In the Colorado River during 1992-1996, spawning began at flows ranging from 8,000-37,000 cfs 1-4 weeks after runoff had peaked for the year and shortly after river temperatures reached 17-18°C (Trammel and Chart 1999x, Anderson 1999). River temperatures were 20-22°C by the time spawning ended (McAda 2001). Although some spawning may occur at cooler temperatures (see Bestgen et al. 1998), most spawning in the Colorado, Green, and Yampa rivers occurs at water temperatures between 18-22°C (McAda and Kaeding 1991, Tyus 1991, Bestgen et al. 1998, Anderson 1999, Trammell and Chart 1999a). Spawning occurs over gravel-cobble substrates in riffles or runs adjacent to pools or low-velocity habitats where adults may stage or rest between spawning efforts (Tyus and McAda 1984). Two canyon-bound reaches in the Green and Yampa rivers are used for spawning by most adults that reside in the Green River system (Tyus 1991, Irving and Modde 2000). In the Colorado River, adults spawn in smaller groups and in more locations (McAda and Kaeding 1991). Over the years, five suspected spawning sites have been located in the Colorado River. All have been in alluvial reaches. Aggregations of adults have been documented at one site in the Grand Valley in three different years (USFWS, unpublished data). At this site, spawning appeared to occur at the base of a chute channel that bisected a cobble-based island. Cobble spilling into the main channel from this side channel was very loose with deep interstitial spaces (Bliesner and Lamarra 1995). Other adults were found in eddies and calm zones between the thalweg and island shoreline, ostensibly staging or resting (USFWS, unpublished data). Colorado pikeminnow are broadcast spawners with adhesive eggs (Hamman 1981), and it is hypothesized that eggs settle into the interstitial voids of cobble substrates. Northern pikeminnow P. oregonensis spawn over similar substrate and eggs have been found 15 cm below the substrate surface (Beamesderfer and Congleton 1982). Colorado pikerninnow eggs incubate for 4-7 days depending on water temperature (Hamman 1981, Marsh 1985, Bestgen and Williams 1994) and larvae remain in the gravel for an additional 6-7 days after hatching (Bestgen et al. 1998). Emerging larvae are entrained in the current and many drift long distances downstream before being deposited in backwaters of low- gradient reaches (Tyus and Haines 1991, Trammell and Chart 1999a). 16