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5 STUDY AREA Field activities were conducted in the Colorado (Mesa County) and Yampa Rivers (Moffat County) (Figure 1). The Colorado River study area includes an approximately 32 km (20 mi) river reach from the vicinity of Loma to the Colorado-Utah state line. The Yampa River study area includes an approximately 95 km (59 mi) reach from the upper end of Cross Mountain r Canyon to the confluence with the Green River. Approximately 75 km (46.5 mi) of the study area are within Dinosaur National Monument. METHODS General. Field sampling was conducted during periods which reflected pre-runoff, peak-runoff, and post-runoff conditions in the Colorado and Yampa study areas. Collections were made during 4-5 day field trips utilizing a power boat and inflatable rafts in the Colorado while rafts and canoes were used in the Yampa. Cross Mountain Canyon was surveyed entirely on foot since the extremely turbulent nature of the canyon made float trips unsafe. Fishes were collected with 3.0 x 1.2 m and 1.0 x 1.2 m seines (1.6 mm square mesh) and dip nets (0.79 mm square mesh). Samples were collected according to the "qualitative representative sample" approach (Hocutt et al., 1974) i.e., all available habitats that could be seined were sampled to obtain a representative collection of species at a given locality and which would provide relative abundance information, recognizing the bias of the collection technique employed. Current velocity was measured with Marsh-McBirney or Pygmy-Gurley flow meters, or float techniques. Water temperature was measured with a Taylor thermometer. In order to correlate fish distribution and relative abundance with the wide range of measurable variables, samples were collected according to a detailed habitat stratification approach which was designed to reflect the geomorphic, hydrological, and ecological variables of the drainages (Appendix A). This approach incorporated methods outlined by Herrington and Dunham (_1967) and Platts (1974), but was modified to reflect conditions of larger streams. In order to interface data with both those collected by DOW personnel in the NW Region and the U.S.F.W.S., habitat designations were coded to be computer compatible with the FWS MANAGE program and data are currently being entered at the Colorado State University Computer Center, Fort Collins. Standardized field/laboratory data forms are employed. Within each study area, three types of sample sites were designated. "Intensive" sections were randomly selected prior to each field trip utilizing a table of random numbers. This process required the initial random selection of one location within the study area, corresponding with a designated river mile. This site, plus each river mile every _ 5 miles (8 km) above and below the random site were designated as intensive sampling sites. At each intensive site, all recognizable habitats- were sampled for a distance of 0.25 (0.4 km) miles above and below the mid-point. Typically, depending upon the initial random site selection, 9-10 intensive sites were designated for the Yampa River study area and 5-6 sites for the Colorado River study area. "Intervening" sections were localities which