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03220&' from alpine in the headwaters, subalpine in the vicinity of Steamboat Springs, to semiarid in the vicinity of the river's mouth, Long-term average annual precipitation decreases from about 23 inches at Steamboat Springs to about 16 inches at Hayden (National Climate Data Center, 1983) to about 12 inches at Maybell (National Weather Service, 1985), The topography is moun- tainous in the vicinity of Steamboat Springs but becomes more hilly with scattered, smaller mountains west of there, The farthest downstream reach of the river is a 45-mile segment through Yampa Canyon in Dinosanr National Monnment, where canyon walls rise hundreds of feet above the river. The Yampa River near Maybell (site 8 in fig, I) transports flow from about the npper two-thirds of the basin (3,410 square miles), This site has the longest period of record for streamflow and water- quality data for the Yampa River. Liebermann and others (1989) report that, for 1950-83, the mean annual streamflow at this site was 1,078,000 acre-ft, averaging I ,490 ft3 is, Their plot of mean daily stream- flow for this period shows snowmelt runoff beginning in late March, peaking in late May at about 7,000 ft3is, and ending in mid-July, The remainder of the year is characterized by base flow that usually is less than 750 ft3is, The Yampa River Basin is sparsely settled, V,S, Census Bureau (2000) estimates for July 1999 indicate populations of about 17,941 persons in Routt County and about 12,714 persons in Moffat County, Most of the population is located along the Yampa River in the eastern one-third of the basin in the towns of Steam- boat Springs (about 7,232 persons in July 1999), a snow-skiing resort during winter; Hayden (about 2,340 persons); and Craig (about 8,853 persons), Outside of these developed urban areas, the predomi- nant industry is coal mining, which is located prima- rily in the tributary drainages south of the Yampa River between Steamboat Springs and Craig, Cattle production is the primary agricultural industry in the study area, Irrigation is not widespread, Sample Collection and Measurements Synoptic samples were collected at nine sites on the Yampa River and at selected tributary sites (fig, 1) during August 16-19, 1999, and during March 13-16, 2000, Synoptic samples were collected to characterize downstream trends in the water quality of the Yampa River. These sampling periods were selected because historical records for the Yampa River near Maybell indicate that measured pH values tended to be largest during August and March, The August 1999 period was chosen to determine maximum fluctuations in measured parameters due to photosynthesis during hot weather and base flow, The March 2000 period was chosen because historical data at Yampa River near Maybell indicate that measured pH values tend to be large during late-winter thaw at lower altitudes and before large discharges result from spring snowmelt at higher altitudes, Tributary sites sampled during August 1999 consisted of three rivers and three creeks; the three creeks were not sampled again in March 2000, To minimize costs and because the emphasis of this study is downstream from Steamboat Springs, tributary creeks npstream from Steamboat Springs were not sampled during either period, During August 16-19, 1999, sites on the Yampa River (except Yampa River at Steamboat Springs and at mouth) were sampled during late afternoon between 1530 and 1615 honrs to assess the maximum effects of photo- synthesis, which commonly canses the largest measured pH; for logistical reasons, the sites at Steam- boat Springs and at mouth were sampled at 1200 hours, For logistical efficiency, tributary sites were sampled between the hours of 1200 and 1325, when the effects of photosynthesis potentially were snbstantial, though not maximum, During March 13- 16,2000, sites on the Yampa River (except for below Stagecoach Reservoir) were sampled between the hours of 1430 and 1530 to assess the maximum effects of photosynthesis, Tributary rivers and the site at Yampa River below Stagecoach Reservoir were sampled at 1200 hours for logistical efficiency, All sites were measured onsite for instantaneous discharge (unless the site was at an active streamflow gage), temperatnre, specific conductance, measured pH, and dissolved oxygen concentration, Discharge measurements and computations were done according to procedures outlined in Rantz and others (1982), In-situ measurements for temperature, pH, and dissolved oxygen concentration were made at the centroid of streamflow, A baffling cylinder was used to prevent streamflow velocity from biasing pH measurements (which are accurate to abont :t:O,03 unit), Samples that were collected for laboratory determinations of dissolved concentrations of major ions, fluoride, silica, organic carbon, ammonia, INTRODUCTION 5