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DESCRIPTION OF STUDY AREA The Colorado River basin is separated into upper and lower basins at a point near Lee's Ferry, Arizona. The upper basin is further subdivided into three major river systems or hydrologic subbasins: the Green, upper mainstem, and the San Juan (Figure 1). The Yampa River, largest tributary to the Green River arises on the western slope of the. Ropky Mountains near Yampa, Colorado. It flows northward and then westerly for approximately 200 mi (320 km) before joining the Green River in DNM. Average annual discharge in the Yampa River is 1.2 million acre feet (Carlson et al. 1979). Flows begin to rise in mid to. late April due to snow melt, increasing from a daily average of 100-500 cfs during August-March to daily averages in excess of 10,000 cfs in June In the winter of 1980-81, precipitation and snow pack were much less than normal and spring runoff in the Yampa River was consequently reduced in both duration and magnitude. The study area included 124 mi (198 km) of the Yampa River and 26 mi (42 km) of the Green River. This area extended from Round Bottom, river mile (RM) 124 (km 198) on the Yampa River at an elevation of 180 m, to its mouth at RM 345 (km 552, elevation 1555 m) on the Green River. The 26 mi (41.6 km) section of the Green River extended down- stream from the Yampa-Green confluence to RM 319 (km 510.4, elevation 1463 m). From RM 124 (km 198) downstream to RM 46 (km 74), the Yampa River meanders slowly through low gradient agricultural valleys. This flow pattern is briefly interrupted at two points: Juniper Mountain, RM 88.7 - 91.0 (km 141.9 - 145.6), and Cross Mountain, RM 55.6 - 58.8 (km 89 - 94.1), where the river has steepsided canyons with boulder/bedrock substrate. Near the lower end of Juniper Canyon is the only manmade impediment to flow within the study area, a low level irrigation diversion formed by bulldozing rubble into the river. The Yampa River enters DNM at RM 48 (km 77) and, at RM 45 (km 72), enters a steep-walled canyon with near-vertical walls which rise to 1370 ft (400 m) above river level. The upper canyon, from RM 45 (km 72) to RM 20 (km 32), is characterized by several long flatwater sections interrupted by whitewater (produced by boulder fields from tributaries). At Rri 20 (km 32), the Yampa enters the Weber sandstone and the boulder/cobble substrate gradually includes increasing amounts of sand. With the exception of Warm Springs Rapid at RP4 4.1 (km 5.6), this section of the lower Yampa River is relatively deep and slow-moving with many meanders. The Green River receives the Yampa at RM 345 (km 552) and abruptly enters Whirlpool Canyon, a section with deep eddies and steep, boulder- strewn cliffs. At RM 334 (km 534) the river leaves Whirlpool Canyon and meanders through cottonwood-lined sand/cobble shorelines in Island Park and Rainbow Park until entering Split Mountain Canyon at RM 327 (km 523). During its passage through Split Mountain the Green River passes through several large boulder fields, which create four major rapids. The river exits Split Mountain Canyon at RM 319 (510 km) and enters a broad flat agricultural valley. 2