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Because seasonal differences in discharge dominate the Yampa River system, sampling schedules were designed to measure conditions during different river runoff stages. U.S. Geological Survey discharge records were utilized to construct discharge hydrographs for 1980 and 1981 (Figure 5). Field work schedules were designed to sample all strata at least once each during prerunoff, runoff, and postrunoff stages. Although field sampling was scheduled for late March, lack of field personnel delayed work for a month. A posteriori inspection of hydrographs confirmed that this delay precluded sampling during runoff on both the Yampa and upper Green rivers (Figure 5). During the present study, the runoff period for the Yampa River was taken as that date when average daily flow (measured at the USGS gauging station, Maybell, Co.) first exceeded 800 cfs and continued to increase. Thus, the runoff period on the Yampa began on April 15, 1981 and continued until June 26, 1981 when average daily flow fell below 800 cfs. Under these criteria, all field sampling on the Yampa River occurred during runoff or postrunoff stage. Runoff on the Green River was taken as the date on which average daily flow measured at Jensen, Utah, first exceeded 3000 cfs and continued to increase. Runoff on the Green thus began on April 19, 1981 and con- tinued until June 18, 1981, when average daily flow fell below 3000 cfs. As with the Yampa sampling, all five sampling rotations on the Green River occurred during runoff or postrunoff stage (Table 3). Physicochemical Description of Strata Physicochemical parameters were measured in order to characterize strata. Average water temperature in the Green River was colder than the Yampa during both runoff and postrunoff. Average water temperature in both rivers generally increased during the year for most strata. Turbidity was consistently much lower during postrunoff than runoff in the four Green River strata and in Strata 1 and 2 on the Yampa River, but were higher during postrunoff in Strata 3 through 8 on the Yampa undoubtedly reflecting high intensity summer storms. Average and maximum depths were higher during runoff for all strata than during postrunoff. The following general narrative description of each stratum is provided as an aid in interpreting physichochemical data (Table 4). Depth and elevations are presented in feet only in this section in order to reduce the confusion of converting to meters. In addition, Figure 6 depicts the occurrence of eight habitat types in the study area. A description of these habitat types is presented in Table 2. The pre- liminary surveys in 1980 indicated the Yampa and Green rivers within the study area were considerably different than the lower Green River. Because of the high degree of heterogeneity of habitats, it was neces- sary to divide the Yampa River into eight strata, or reaches of rela- tively homogeneous fish habitat, and the Green River into four strata. Stratum 8 began at. Round Bottom, RM 124.0 (km 198.4) at elevation 6080 ft and extended to RM 91.0 (km 145.6),at tt? upper end of Juniper Canyon. Depths averaged 5 ft during runoff and 2 ft during postrunoff.- 18