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• i 92 The Yampa River, largest tributary of the Green River, arises on the western slope of the Rocky Mo-dntains near Steamboat Springs, Colorado. It flows westerly and joins the Green River in Dinosaur National Monument (Fig. 1). Within the monument, the lower 72 km of the Yampa River are contained within a steep-walled canyon with near-vertical walls rising to 400 m above river level. The upper canyon, from km 72 to km 32 is characterized by long flatwater sections interrupted by whitewater produced by boulder fields carried in from tributaries. The Yampa enters the Weber sandstone at km 32, where the substrate changes from boulder/cobble to sand. With the exception of Warm Springs rapid at km 6.7, the lower Yampa is relatively deep and slow-moving, with many meanders cut through the soft sandstone cliffs. The Green River receives the Yampa River at km 552 and abruptly enters Whirlpool Canyon, a section with deep eddies and steep, boulderstrewn cliffs. At km 534, the Green River leaves Whirlpool Canyon and meanders through cottonwood-lined sand/cobble shorelines in Island Park and Rainbow Park until entering Split Mountain Canyon at km 523. During its passage through Split Mountain the Green River passes through several large boulder fields creating four major rapids. The Green River exits Split Mountain Canyon at km 510 and enters a broad, flat agricultural valley. Homogeneous river sections, or strata, were delineated from topographic and geologic maps, aerial surveys, and on-ground reconnaissance. Eight strata on the Yampa River (two strata within DNM) and four on the Green River were identified (Fig. 1). Sample sites consisted of randomly selected 0.8 km long sections of river. All habitat types within a sample site were intensively sampled with trammel nets, seines, and electrofishing gear during prerunoff (April), runoff (May), and postrunoff (June through October) for a total of five sampling rotations. In addition, electrofishing was continued downstream for 4 km. Sampling and tagging procedures followed those of Archer et al. (1980). Total effort for the three gear types for each stratum was comparable during the study (Table 1). A total of 20 fish species was found in the Yampa and Green rivers within DNM (Table 2). Seventeen species were common to both rivers. Incidental species included one Ictalurus melas from the Yampa and one Micropterus dolomieui and one Catostomus commersoni from the Green. Seventeen adult Colorado squawfish (432 mm TL to 680 mm TL) were collected from the Green and Yampa rivers within the monument. Although collected from each stratum, squawfish were uncommon. Only five individuals were collected in the lower Yampa canyon (stratum Yl) during standardized sampling; however, an additional 32 were collected and tagged during special spawning studies between 3 July and 10 July. The four Xyrauchen texanus collected in the Yampa River were obtained by electrofishing from a riffle 0.8 km upstream from the Yampa/Green River confluence where several others were observed but not caught. The only X. texanus found in the Green River was collected with electrofishing gear in May, approximately 1.6 km downstream from the confluence. Five humpback chubs were collected in this study. Two were collected with floating trammel nets at km 29 on the Yampa during Colorado squawfish studies in July. One humpback chub was caught in a stationary trammel net at km 26 in September. Two individuals were also taken in August with stationary trammel nets in Whirlpool Canyon.