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Habitat Use and Streamflow Needs of Rare and Endangered Fishes, Yampa River, Colorado by Harold M. Tyus and Catherine A. Karp U.S. Fish and Wildlife Service Colorado River Fishes Project 1680 W. Hwy. 40, Rm. 1210 Vernal, Utah 84078 Introduction The Yampa River -largest tributary to the Green River-is located in northwestern Colorado and southern Wyoming (see map opposite page). From its headwaters on the western slope of the Rocky Mountains (about 3,780 m above sea level; see appendix for measurement conversions), the Yampa River meanders northward and then westward to Craig, Colorado (river kilometer [RK] 224). It then passes through low-gradient agricultural valleys and enters the canyons of Juniper Mountain (RK 145.6-141.9) and Cross Mountain (RK 94.1-88.9). Downstream of the Little Snake River confluence (RK 81.6), the Yampa River enters Dinosaur National Monument (DNM) and the Yampa Canyon (Deerlodge Park to Echo Park, RK 75.2-0), where it becomes ahigher-gradient system of rocky runs and rapids. The gradient again becomes more moderate below Big Joe Rapid (RK 38.4), and the Yampa River deepens and becomes relatively slow-moving through the incised meanders of Mathers Hole (RK 28.3). After a short drop at Warm Springs (RK 6.4), the Yampa River enters the Green River at Echo Park, DNM (about 1,524 m above sea level). Thus, the mainstream Yampa River is composed of relatively high-gradient canyon reaches dominated by boulder, cobble and gravel substrates, and low-gradient reaches of meandering canyon and flat, open terrains dominated by finer substrates. The Yampa River flows about 320 km and drops in elevation about 2,256 m (Joseph et al. 1977). Average annual discharge of the Yampa River is about 60.5 m3/s, of which about 28% is contributed by the Little Snake River (sum of United States Geological Survey [USGS] flow records for Yampa River near Maybell, Colorado, and Little Snake River near Lily, Colorado, for 1922-87, M. Butler, written communication). F1owsbegin torise inthe Yampa River in late March due to spring runoff and remain high through July (Fig. 1). Mean flow during~spring runoff in the Yampa River is about 153.18 m /s (USGS flow records). River level may undergo large fluctuations during spring runoff due to local warming trends and flash floods. Although floods are of short duration, peak flows can be high. A maximum discharge of 939.56 m3/s was recorded 18 May 1984 in the Yampa River at Deerlodge Park (Ugland et al. 1987). Following spring runoff, flows of the Yampa River decline toward a monthly base flow of about 14.15 m3/s for August through March (USGS flow records), and large fluctuations in river level are infrequent events. Thus, fishes indigenous to the Yampa River evolved in a system of fluctuating seasonal and annual flows characterized by wet, average, and dry climatic periods. We consider the recurring cycle of high spring flows followed by a period of lower flows (Fig. 1) the natural or current flow regime of the Yampa River. This regime is typically a system of fluctuating low, average, and high flow years. 351 30( xsc zoc = 15C U ~ 100 50 Fig. 1. Average annual distribution hydrograph, Yampa River, 1922-87. +•^~••~ oumme gUTUMN WINTER SEASON