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Measurements in the Yampa and the Little Snake Rivers were made to determine traveltime, unit-concentration, and longitudinal-dispersion characteristics of upstream reaches during highand low-streamflow conditions. Reaeration coefficients were measured for the Yampa River, using a modified tracer technique to quantify the process of reaeration by which the stream replaces the dissol^d oxygen consumed in the oxidation of organic wastes. Stream reaches studied included a 58-mile (93~kilometer) reach of the Yampa River and a 77~mile (124-kilometer) reach of the Little Snake River. Field data were collected during June and September 1976, and May 1977Two traveltime measurements using a fluorescent dye were made on the Yampa River when discharges were approximately 100 and 3,400 cubic feet per second (2.8 and 5.2 cubic meters per second), and three traveltime measurements were made on the Little Snake River when discharges were approximately 20, 600, and 1,600 cubic feet per second (0.56, 16.8, and 44.8 cubic meters per second). Measured stream velocities varied as follows: 0.26 to 3.6 miles per hour (0.42 to 5.8 kilometers per hour) for the Yampa River and 0.04 to 3.5 miles per hour (0.06 to 5.6 kilometers per hour) for the Little Snake River. Simulations of traveltime and unit concentrations for the Little Snake River were made using a mathematical model. Medium-flow data (600 cubic feet per second or 16.8 cubic meters per second) were used as a check of the model accuracy. Traveltime simulations compared within 5 percent, and unitconcentration simulations were within 30 to 40 percent of the measured flow data. An analysis of the waste-load assimilative capacity of the Yampa River, completed earlier, used traveltime estimates based on cross-sectional properties of streams and velocity and linear-regression relationships. A comparison of the traveltime simulations from this study agreed within 6 percent of the estimates from the waste-load analysis. Longitudinal-dispersion coefficients were computed for lowand highflow conditions in the Yampa River and for mediumand high-flow conditions in the Little Snake River. Longitudinal-dispersion coefficients ranged from 400 to 6,050 square feet per second (37.2 to 560 square meters per second) for the two rivers. Reaeration coefficients measured for low-flow conditions on the Yampa River, and adjusted to 20 degrees Celsius, ranged frotti 6.04 to 33-4 day' 1 . Two semiempirical (energy-dissipation) equations gave coefficients in best agreement with the measured coefficients. Absolute erjrors of estimate for these equations were 11.8 and 17-3 percent. Management activities that could use results of this study include predicting arrival time and concentration of soluble contaminants accidentally spilled in streams, length of stream affected by a municipal wastewater discharge, and traveltime required for reservoir water released for given downstream needs. INTRODUCTION The Yampa River basin is undergoing substantial economic development, spurred predominantly by mining, transport, and conversion of coal resources in the basin. The impacts of this development pose existing and potential stresses on the basin's limited water resources. This report is part of a series of multidisciplinary studies conducted by the U.S. Geological Survey in the Yampa River Basin Assessment project (Steele and others, 1976a, 1976b). The multidisciplinary studies include a wide range of existing conditions and anticipated changes in the availability and uses of the basin's water resources (Steele and others, 1979). Impending population growth resulting from energy development and greater recreational use in areas along principal streams of the Yampa River basin may increase the discharge of wastes to these streams. A knowledge of existing streamflow conditions will aid State and local officials in determining how fast wastes move downstream, how they are dispersed vertically and horizontally in streams, and how rapidly streams can assimilate certain forms of treated wastes (Bauer and others, 1978). Purpose and Scope The first purpose of the study was to determine! traveltime, unitconcentration, and longitudinal-dispersion characteristics for specified reaches of the Yampa and the Little Snake Rivers for a I' range of streamdischarge conditions. A second purpose was to measure reaeration coefficients (#2) for three reaches of the Yampa River and then to compare measured #2 values with those computed using various empirical equations. Studies were conducted on the following stream reaches: (1) The Yampa River from about 5 mi (8 km) southeast of Steamboat Springs, Colo., downstream to Craig, Colo., and (2) the Little Snake River from 16 mi (26 km) east of Slater, Colo., downstream to 33 mi (53 km) southwest of Baggs, Wyo. (fig. 1). Studies of the Yampa River included determinations of travel time, unit-concentration, longitudinal-dispersion, and reaeration coefficients. The study of the Little Snake River included only determinations of travel time, unit concentration, and longitudinal dispersion. This report describes results of an analysis of two sets of traveltime, unit-concentration, and longitudinal-dispersion data for the Yampa River and three sets for the Little Snake River. Determinations of traveltime were made for high-flow (June) conditions to low-flow (September) conditions. These data were used to develop approximate relationships of traveltime and unit concentration versus discharge for each of the stream reaches. Longitudinal-dispersion coefficients were also determined for each of the stream reaches. Reaeration coefficients were measured for low-flow (September) conditions in the Yampa River study reach, using a modified tracer technique. The measured reaeration coefficients were compared with various empirically determined coefficients and the equation providing the best comparison was determined. Acknowledgments The authors thank the many residents along the Yampa and the Little Snake Rivers for permitting access to stream sites during periods of fielddata collection for these studies. The authors also are grateful to Philip E. Stark, Routt County Department of Environmental Health, and David J. Shultz, U.S. Geological Survey, for assistance in the planning and collection of the data. The contributions of James F. Kircher, U.S. Geological Survey, Cheyenne, Wyo., in the preparation of this report are gratefully acknowledged. Assistance in collecting the data provided by personnel of the Colorado and Wyoming offices of the U.S. Geological Survey is appreciated. Laboratory assistance of Doreen Y. Tai of the U.S. Geological Survey in the determination of the tracer-gas concentrations for the reaeration study is also gratefully acknowledged. LOCATION AND EXTENT OF STUDY REACHES The general location and extent of the study reaches of the Yampa and the Little Snake Rivers are shown on figure 1. The length of the study reach along the Yampa River is 58 mi (93 km), and the study reach along the Little Snake River is 77 mi (124 km). Data used for the studies reported here were collected during June and September 1976, and May 1977During September, flow conditions for the two rivers generally can be characterized as low with approximately 90-percent duration (flow duration is defined as the percentage of time the flow was equaled or exceeded), and as high during June with approximately 10-percent duration. During September, only 20 to 30 percent of each stream reach was sampled, due to the low streamflow velocities. A third E X P L A N A T I O N | f S T U D Y R E A C H E S

water-resources investigations report

Traveltime, unit-concentration, longitudinal-dispersion, and reaeration characteristics of upstream reaches of the Yampa and Little Snake Rivers, Colorado and Wyoming