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t 1 1 L 1 1 t 1 Chapter 1 Introduction BACKGROUND Hydrologic Setting The Yampa River drains an area of approximately 9,500 square miles in northwestern Colorado and south-central Wyoming and contributes an average of roughly 1.5 million acre- feet (maf) of flow annually to the Green River at the Echo Park confluence in Dinosaur National Monument. Above the confluence of the Little Snake River the benchmark stream gage is the Maybell gage just east of the town of Maybell, Colorado. Figure 1-1 shows the Yampa River basin above the Little Snake River and the locations of some of its principal tributaries and other features. For reasons explained more fully below, the focus of this report is on the Yampa River mainstem between 'the Little Snake River and the town of Mt. Harris. Much of the flow of the Yampa River is the result of snowmelt from the eastern and southern edges of the basin. Accordingly river flows show great seasonal variation. As shown in Figure 1-2, recorded daily flows at Maybell range over four orders of magnitude from 2 cfs to 24,400 cfs (similar graphs for other gages in the basin can be found in Appendix A). About two-thirds of the 1.1 maf average annual flow at the Maybell gage occurs during May and June. Annual water consumption in the basin is estimated to be approximately 110,000 of/yr, or about 10 % of the flow at Maybell. By far the largest consumer of water is irrigation; there are hundreds of irrigation ditches on the mainstem and on tributaries diverting water to upwards of 75,000 acres of irrigated farmland. The largest single irrigation diversion is the Maybell Canal, which takes roughly 10,000 of/yr from the river just upstream from the Maybell gage. Endangered Species Because of its relatively undeveloped state, the Yampa River is believed to contain some of the largest remaining habitat of four endangered native Colorado River fishes---the Colorado River Squawfish (Ptychocheilus Lucius), the Humpback Chub (Gila cypha), the Bonytail Chub (Gila elegans), and the Razorback Sucker (Xyrauchen texanus). Designated critical habitat for the Colorado River Squawfish extends upstream on the Yampa to the western edge of the City of Craig and occupied habitat is believed to!, extend further upstream, possibly as far as the town of Hayden. While there is not unanimous agreement among fisheries biologists, it is generally thought that local adult squawfish populations travel downstream during spring runoff to spawn on gravel bars in the lower reaches of the river. When spawning is completed and runoff begins to recede, they begin to migrate back upstream to their over-winter habitats. This upstream migration can extend from late summer to the fall months. 1-1