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Within each 0.5 mi (0.8 km) station, discrete habitat types were delineated (Figure 3) and fish collections were conducted within them, rather than across several habitats. This permitted habitat preference delineation for fish species with depths, velocities and substrates recorded for each collection. Sampling was stratified temporally by day and season. Days were subdivided into three sampling periods, and a midday period of sampling inactivity was reserved for equipment maintenance, travel and personal activities of the crew (Figure 4). Daylight activities were devoted to netting, trapping, electrofishing and physical/chemical measurements. Activities during darkness were devoted to tending nets, traps, elec- trofishing and some seining. Seasonal sampling conformed to river stage. During 1981 only runoff and postrunoff river stages were sampled in each of the 12 strata (4 on the Green River; 8 on the Yampa River). Standardized sampling schedules and procedures were closely adhered to; however, weather conditions, manpower shortages, and equipment failures at times dictated minor changes in schedules and sampling procedures. Sample sites were precisely located on topographic maps. Sketches of habitat locations within each 0.5 mi (0.8 km) station were made according to the categories listed in Table 2. Three cross section depth profiles were recorded in feet at each 0.5 mi (0.8 km) station using a sonic depth recorder. Depth profiles in feet were recorded at each end of the 0.5 mi (0.8 km) station and at the midpoint of the section. Water quality measurements were taken at each station. These included dissolved oxygen (DO), turbidity, air and water temperatures, salinity and conductivity. Such variable parameters as temperature and DO were taken between 1000 and 1400 hours for standard- ization. Five survey stations were also established on the Yampa for the purpose of modeling hydraulic and physical habitat conditions in repre- sentative reaches of the Yampa to these general physical habitat surveys (Appendix A). An effort was made to collect fish from every habitat present in a sample station. The habitat from which fish were collected was described by a series of depth, velocity and substrate measurements. Bottom substrates were classified according to the following: Silt Sand Gravel Rubble Boulder Bedrock fine gritty material less than 3 mm diameter 3 mm to 76 mm 76 mm to 305 mm 305 mm and larger rock outcropping 8