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Summer 1999 Data collection during summer 1999 consisted of synoptic sampling at nine Yampa River sites and six tributary sites (fig. 1), diurnal measuring at five Yampa River sites (fig. 1), and reconnaissance measuring of the Yampa River in Dinosaur National Monument between Deerlodge Park and the mouth of the river (Echo Park). Synoptic Sampling Synoptic samples were collected during August 16-19, 1999, at the sites listed in table 1 (and at three small creeks discussed in this section). Water-quality data resulting from this sampling are presented in table 3 in the "Supplemental Information" section at the back of this report. Samples generally were collected in the afternoon during a period of relatively low stream discharge and high temperature to assess the effects of photosynthesis, particularly on pH. Because these synoptic samples were collected over a 4-day period that was not substantially affected by rainfall, they reasonably represent a "snapshot" view of water- quality conditions along the Yampa River during after- noon hours. Based on measured instantaneous discharges, the three main tributaries (fig. 1) augmented flow in the Yampa River at their confluences approximately by the following percentages: Elk River, 77; Williams Fork, 23; and Little Snake River, 26. In contrast, the Table 1. River miles of sites sampled for this study next three largest tributaries downstream from Steam- boat Springs augmented flow in the Yampa River approximately by the following percentages (based on discharges at nearest upstream Yampa River site or sum of discharges of the nearest upstream Yampa River site and nearest upstream tributary site): Trout Creek, 6.8; Elkhead Creek, 1.3; and Milk Creek, 2.1. Because of the insignificant potential of these creeks to affect water quality of the Yampa River, they will not be discussed further in this report. During the synoptic sampling, specific conduc- tance (fig. 3) decreased substantially from 419 µS/cm at Yampa River below Stagecoach Reservoir (site 1 in fig. 1 and table 1) to 294 µS/cm at Yampa River at Steamboat Springs (site 2) due to dilution with fresher water in tributary creeks, which substantially increased discharge from about 99 ft3/s at site 1 to about 160 ft3/s at site 2. Specific conductance increased slightly to 320 µS/cm at Yampa River above Elk River (site 3), which is about 1.8 miles down- stream from the outfall from the Steamboat Springs Regional Waste Water Treatment Plant (SSRWWTP). Substantial dilution with fresher water (specific conductance of 102 µS/cm) from Elk River near mouth (site 4) caused a substantially smaller specific conduc- tance of 260 µS/cm at Yampa River below diversion near Hayden (site 5). Downstream from site 5, specific conductance increased gradually to 557 µS/cm at Yampa River at mouth (site 12), primarily because of substantial contributions of saltier water from Site number in Distance from mouth figures 1, U.S. Geological Survey site identification of Yampa River, in 3~, and 13-15 river miles 1 Yampa River below Stagecoach Reservoir, 09237500 221.0 2 Yampa River at Steamboat Springs, 09239500 199.1 3 Yampa River above Elk River, 402936106565000 191.8 4 Elk River near mouth, 402914106580400 1190.1 5 Yampa River below diversion near Hayden, 09244410 177.2 6 Yampa River below Craig, 09247600 140.0 7 Williams Fork at mouth, 09249750 1135.1 8 Yampa River near Maybell, 09251000 89.0 9 Yampa River above Little Snake River, 09251100 53.0 10 Little Snake River above Yampa River, 402925108253200 151.1 11 Yampa River at Deerlodge Park, 09260050 46.0 12 Yampa River at mouth (Echo Park), 403136108585900 0.0 Distance is from mouth of tributary river INTERPRETATION OF DATA COLLECTED FOR THIS STUDY