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sampled via boat so sampling efforts were concentrated in the 0.5 mi (0.8 km) stretch at RM 53.0 (km 84.8). A PHABSIM site was established in this stratum at RM 53 (km 45) (Appendix A). Stratum 3, Deerlodge Park, was a very short stratum which extended from the mouth of the Little Snake River RM 51.0 (km 81.6), downstream to RM 45.0 (km 72) where the river abruptly entered Yampa Canyon. PHABSIM surveys were also conducted within this stratum at RM 50.(km 80) (Appendix A). Average gradient was only 3.3 ft/mi and the substrate was predominantly sand/silt, reflecting high sediment inputs from the Little $Aake River. This was the most shallow stratum, averaging only 2.0 ft deep during runoff and 1.0 ft during postrunoff. Habitat diversity was poor; only a few backwaters and side channels occurred. As with strata 7, 5, and 4, sampling effort was concentrated in one index station. RM 51,0 to RM 49.0 (km 81.6 to km 78.4). Stratum 2, upper Yampa Canyon, from RM 45.0 (km 72) to Harding Hole at RM 20.0 (km 32.0), consisted of long slow-moving sections interrupted by boulder fields deposited from tributary flooding. Substrate was mainly gravel/cobble with some areas of sand. Named rapids occurred at four of the boulder fields. Average gradient for the entire stratum was 15.9 ft/mi, but was much greater than this (31.0 ft/mi) in a short stretch at Teepee Rapid, RM 36.4 (km 58.2). Canyon walls were in excess of 1500 ft above river level. Stratum 1, lower Yampa Canyon, extended from Harding Hole at RM 20.0 (km 32.0) to the Yampa/Green confluence at Echo Park, RM 0.0 (km 0.0). Substrates were predominately composed of sand but substantial areas of gravel/rubble occurred. Average gradient was 6.4 ft/mi. Except for Warm Springs Rapid at RM 4.0 (km 6.4), the lower Yampa is relatively slow-moving, with many meanders cut through the soft sand- stone cliffs, which rise to heights of 1700 ft above river level. Two PHABSIM stations were established in this reach, one at RM 16.5 (km 21.5) and one at RM 2 (km 3.3) (Appendix A). At Echo Park the Yampa River joins the Green River. Ongoing CRFP studies in 1979-81 had designated the Green River from the mouth of the Yampa, RM 345.0 (km 552), downstream to Split Mountain, RM 319.0 (km 510.4), as Stratum G. When the Yampa River Study began in the spring of 1981, this stratum was further subdivided into four distinct strata (designated G1, G2, G3, G ,) for sampling purposes. The Green River is controlled by releases frK Flaming Gorge Dam and unpredictable water level fluctuations of several feet may occur daily. Stratum G4, Echo Park, extended for only 2 mi from the mouth of the Yampa River downstream to RM 343.0 (km 549). Average width of the river is almost 400 ft (120 m), and average gradient is only 3.5 ft/mi. Largely because of accessibility by road, the Green River (and the lower Yampa) at Echo Park has been relatively well-sampled by.several agencies and individuals. Substrate was a mixture of sand/silt and several large gravel/cobble riffles. Habitat types included several backwaters and large eddies. Since this stratum was smaller than our standard 2.5 mi (4 km) stations, a decision was made to treat it as an index station. 25