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samples and reduce within-strata sampling variability. Criteria incorpo- rated into the delineation of strata included both geophysical (river • channel configuration, substrate types, gradient, and depth) and biolog- ical parameters (productive bottomlands, sterile sand flats, productive backwaters, swift canyons, and point sources of allochthonous organic materials). The six homogeneous strata were as follows: Len gth kilo- River Stratum miles meters milesl/ Landmark description A 71 114 222291 Turks Head to Labyrinth Canyon B 37 59 94-131 Labyrinth Canyon to Gunnison Butte C 24 38 132-156 Gunnison Butte to Three Ford's Rapid D 54 86 157-211 Three Ford's Rapid to Duche's Hole E 37 59 212-249 Duche's Hole to Duchesne River F 70 112 250-320 Duchesne River to Split Mountain 1/ River mile from confluence of the Green and Colorado Rivers. Sampling design and procedure Sampling procedures were presented in Archer et al. (1979). During the first year a random sampling design was utilized to establish the sampling sequence. One stratum (B) was selected as the starting point with the sampling series progressing sequentially downstream to the last stratum, shifting back to the uppermost stratum to complete the series. Within each stratum sample stations were selected according to a random sampling design. In order to avoid sampling bias in the selection of stations a table of random digits was consulted and the first number encountered in the table that appeared in the section was chosen for the first sample station. To permit replication and to insure that representative stations were chosen, each station consisted of two 0.5-mile (0.8-kilometer) reaches; one extending downstream from the selected river mile starting point, the other 5 miles (8 kilometers) downstream (Figure 3). Standardized sampling began in the uppermost 0.5-mile (0.8-kilometer) reach continuing for 24 hours. The second day the intervening 4.5-mile (7.2-kilometer) reach was surveyed by electro fishing starting at the lower boundary of the previous sample station and continuing downstream to the upstream point of the second 0.5-mile (0.8-kilometer) station. From this point the second 0.5-mile (0.8- kilometer) station was intensively sampled for 24 hours. Within each 0.5-mile (0.8-kilometer) station discrete habitat types were delineated, (Figure 4) and fish collections conducted within them rather than across several habitats. This permitted a delineation of habitat preferences for fish species with depths, velocities, and substrates recorded for each subsample. • Sampling was not only stratified spatially, but also temporarily by day and season. Days were subdivided into three sampling periods 6