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OJJ076 National Wild and Scenic Rivers Recreation, written commun., reconnaissance survey described water hydrologic information Dinosaur National monument. System (H. J. Belisle, U.S, Bureau of Outdoor 1976; U.S. National Park Service, 1977). The here was undertaken to obtain basic surface- during low flow for the lower Yampa River in Ground-water resources of Dinosaur National monument have been studied previously by Sumsion (1976). He found that all water used for public supply within the monument is from wells, and that most of the ground water is obtained from limestone and sandstone formations, ,such as~theMorgan Formation and the Weber Sandstone (fig, 2), which yield a maximum of 0.6 to 1.3 m3/s (cubic meters per second) to springs and wells in the monument. Alluvium along the major stream channels is the source of small amounts of water to wells, but some of this water is not of suitable chemical quality for public supply (Sumsion, 1976). Most of the ground water obtained from the limestone and sandstone aquifers is suitable for publ ic supply. Surface-water resources of Dinosaur National monument were evaluated with respect to quantity by Sums ion (1976); however, the study reported here is the only known work that includes an assessment of surface-water quality in the monument. HYDROLOGIC RECONNAISSANCE The hydrologic reconnaissance was conducted during August 1976 by R. C. Averett, R. E. Brogden, J. F. Gibbs, R. J. Pickering, T. D. Steele, and D. A. Wentz, of the U.S. Geological Survey. Transportation, guides, and supplies were provided by Adventure Bound, Inc. Permission to make the float trip on the lower Yampa River was granted by C. D, Lewis, Jr., Superintendent, Dinosaur National monument, and is gratefully acknowledged. The reconnaissance began on August 17, 1976, at Deerlodge Park and ended on August 19 at Echo Park, where the Yampa River enters the Green River (fig. 1). The length of the study reach was approximately 74 river kilometers, and altitudes of the sampling sites ranged from about 1,710 m (meters) at the most upstream site to 1,540 m at the most downstream site (U,S. Geological Survey, 1966). River positions were determined from topographic and river-profile maps (U.S. Geological Survey, 1922; 1966). The stream generally consisted of a pool-and-riffle regime, with pools often exceeding 2 km (kilometers) in length. The reconnaissance was completed by rubber rafts, and, in numerous areas, the rafts had to be pulled over the shallow riffles. Stream-bottom materials varied from large boulders and cobbles to fine, shifting sands and silts. The two dominant geologic formations forming the canyon walls of the lower Yampa River in Dinosaur National monument are the Morgan Formation and the Weber Sandstone (fig. 2)" which are both of Pennsylvanian age. The Morgan Formation is a 30- to 460-m thick I imestone interbedded with shales and sandstones. The Weber Sandstone is an approx,imately 300-m thick fine-grained 3