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3 STUDY AREA Field activities were conducted in the Colorado, Yampa, and Green rivers (Mesa and Moffat Co.), Colorado (Fig. 1). The Colorado River study included a 32-km reach from Loma to the Colorado-Utah state line. The Yampa River study area included a 95-km reach from the upper end of Cross Mountain Canyon to the Yampa-Green River confluence at Echo Park. Approximately 75 km of the Yampa River study area (from Deerlodge Park to Echo Park) was within Dinosaur National Monument (DNM). An additional site on the Green River at the Colorado-Utah state line in DNM (approximately 9.3 km below the Yampa-Green River confluence) was included in 1984. METHODS Field sampling for larvae and early juveniles was conducted during spring through late summer/early fall in 1979-1983, 1980-1984, and 1984 in the Colorado, Yampa, and Green rivers, respectively. During 1981-1983, sampling was conducted at periods which reflected pre-, peak, and post-runoff flow regimes (1979-1980 collections were largely opportunistic; 1984 collections in the Yampa and Green rivers were made only during post-runoff flow). Collections were made during several 4-5-day field trips. Power boats and/or inflatable rafts were used on the Colorado River, while rafts and canoes were used on the Yampa and Green rivers. Cross Mountain Canyon was surveyed entirely on foot since the extremely turbulent nature of the Yampa River within this reach made float trips unsafe. Seine sampling locations within a given study area were, in part, determined using a stratified random design during 1981-1984 and samples were collected in a consistent manner at each individual locality. Three types of sampling sites were designated. (1) "Intensive" sites were randomly selected prior to each field trip using a random numbers table. This process required the initial random selection of one location within a study area, corresponding to a river kilometer. This site, plus each river kilometer approximately 8 km above and below, was designated as an "intensive" site. At each "intensive" site, all recognizable (and seinable) habitat types were sampled for a distance of 0.8 km above and below the mid-point. Depending upon the location of the initial random site determined, the number of "intensive" sites sampled per field trip were 5-6 and 9-10 for the Colorado and Yampa rivers, respectively. (2) "Intervening" sites were localities excluded from the random site selection but sampled due to the wide variety of habitat types available. (3) "Special" sites (e.g., Black Rocks, river km 219.8-218.2, Colorado River) were relatively short, unique river reaches or points included for specific purposes.