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QDI7i:. centrations at various downstream locations for a contaminant discharged or spilled upstream. Traveltime and unit concentrations for a range of stream-discharge conditions were estimated using the measured data and a mathematical model described by McQuivey and Keefer (1976). The simulation of traveltime and unit-concentration data involves initially calibrating the model for the observed data and then simulating results to other flow conditions. Cumulative traveltime and unit- concentration relationships can then be derived for any stream reaches at any assumed discharge (Bauer and others, 1979). Traveltime simulations for the Little Snake River using the model-analysis scheme were compared and agreed within 5 percent of I. he measured medium-flow data for May 1977. A similar comparison also was made for the unit- concentration data. with agreement within 30 to 40 percent. Traveltime simulations based on linear- regression relationships of mean stream velocity or discharge also are given for both streams (Bauer and others, 1979). Longitudinal-dispersion coef- ficients. computed for the Yampa and the Little Snake Rivers, ranged from 6.050 to 400 ft'/s (560 to 37 m'/s) for the two streams. As an example of an application of the derived traveltime information, assume that an accidental spill occurred in the Steamboat Springs area in which 1,000 pounds (453 kg) of a soluble conser- vative contaminant was spilled into the Yampa River. Assume a stream discharge at Steamboat Springs (fig. 2) of 1,000 ft'/s (28 m3/s). One im- mediate concern might be to determine the arrival time and peak concentration of this contaminant at the Hayden water plant downstream. At this discharge, the traveltime of the leading edge of the conservative contaminant could be expected to ar- rive at Hayden in approximately 12.5 hours. The estimated peak concentration would be 1,580 f.Lg/L at Hayden, 15.7 hours after the spill (Bauer and others. 1979). Health standards for a particular contaminant would indicate the potential toxic ef- fects of such a concentration and dictate whether or not the water use should be curtailed. In conclusion, prediction of travel rates and dispersion in streams is important for pollution control and for use as a warning system in case of contaminant spillage upstream from critical water- use diversion points. The technique described above provides a means of estimating stream- travel rates and unit concentrations for a large range in flow conditions, based on one field- sampling study. In cases where no traveltime studies can be made, a regional regression relationship may be used (Boning, 1974). In order to meet projected future demands for water, considerable interest has been expressed and plans proposed for additional development of surface waters of the Yampa River basin. As in- dicated by Maddock and Matalas (1980), some of the future water demands involve year-round use for cooling systems in coal-conversion facilities. Also, anticipated population growth results in in- creased household and service-related water de- mands. Streamflow in the basin presently is regulated by few reservoir impoundments. Thirty- five major reservoirs with storage capacities larger than 2.000 acre-ft (2.5 million m3) have been proposed as part of 18 surface-water projects (Steele, 1980), The total potential reservoir capacity is approximately 2.2 million acre-ft (2,7 billion m'), which is 41 percent more than the mean annual outflow from the basin. Feasibility and planning reports for several of the proposed reservoirs have been prepared (Oak Creek Power Co.. 1976; U.S. Department of the Interior. 1976b). Two projects to export substantial amounts of water for use outside the basin also have been proposed. These include the Vidler Tunnel diver- sion involving tributaries of the Yampa River and the Hog Park diversion involving tributariea of the Little Snake River (fig. 2). Within this particular investigation. three modeling efforts were conducted interactively as a demonstrat.ion study to evaluate various aspects of proposed reservoir development and the tran- smountain diversions (D. B. Adams, 0, P. Bauer, R. H. Dale, and T. D. Steele, written commun.. 1978). A reservoir-system model, HEC-3 (U.S. Army Corps of Engineers, 1968). was used to analyze effects on streamflow of alternative reser- voir configurations described below. A river- salinity model (Ribbens. 1975) was used to es- timate dissolved-solids concentrations at selected locations in the basin for the different reservoir configurations. A third model, using the streamflow and water-quality information provided in part by the first two models, evaluated inreservoir water temperatures and specific- conductance conditions at selected sites for as- 29