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002551 The profile for average concentrations of CBODs also is shown on figure 7. Except for a sub reach downstream from the main wastewater-treatment plant at Steamboat Springs (site YE-3) and another subreach downstream from the Elk River (site YT-9) near ,Milner, Colo. (table 2), the, concentrations of CBODS are less than 3.0 mg/L. The larger values downstream from Steamboat Springs were expected because of the effects of waste discharges, but the relatively large values downstream from the Elk River are not readily explained. One possible cause of the larger concentrations of CBODs downstream from the Elk River is irrigation return flow in this area of the study reach. While the 24-hour survey was being completed, small volumes of return flow in this area were noted, but no samples for analysis of BOD were collected. The concentrations of total-coliform and fecal~coliform bacteria along, the study reach are shown on figure 8. As shown, the higher coliform-bacteria concentrations occur downstream from the Steamboat Springs area where the larger proportion of treated-wastewater effluents discharge into the Yampa River. The two profiles, in general, indicate a gradual die-off of coliform bacteria with increasing distance downstream from Steamboat Springs. The reason for the larger concentration of fecal-coliform ,bacteria in the vicinity of site YM-12 is not known. Variations in concentrations of total-coliform and fecal-coliform bacte- ria for the 24-hour sampling period at site YM-4 are shown on figure 9. Site YM-4 is located downstream from the effluent discharge of the main wastewater- treatment plant at Steamboat Springs (fig. 6). The large coliform-bacteria concentration at the site is the result of the upstream discharge of waste- water effluents. Concentrations of total-coliform bacteria varied consider- ably during the 24-hour sampling period; for example, coliform bacteria at site YM-4 (fig. 9) ranged from 380 to 3,500 colonies/100 mL (milliliters) of water. The diel variations of DO concentrations are shown on figure 10 for sites YM-2, YM-6, and YM-11 (fig. 6 and table 2). A general cyclical pattern is observed at all three sites, with highs during daylight hours typifying photosynthesis in the stream, and with lows at night ind,icating relative effects of respiration. The largest diel variation (7 to 10.5 mg/L) occurred at site YM-6, which is located approximately 5 mi (8 km) downstream from the discharge outlet of the main wastewater-treatment plant at Steamboat Springs. Site YM-2 is located upstream from this outlet. A diel variation of 8.2 to 10.2 mg/L in DO concentration occurred at site YM-2 during the 24-hour cycle. Data at this site include the effects of the discharge from the wastewater- plant at Mount Werner and any nonpoint sources from Steamboat Springs. Site YM-11 is located upstream from Sage Creek on the Yampa River (fig. 6). A diel variation of 7.3 to 9.7 mg/L in DO concentration occurred at this site during the 24-hour cycle. Along the entire length of the study reach, and especially in sections containing pools, varying concentrations of floating and attached plants were observed. In some subreaches, these plant concentrations were larger than in others, with the largest concentrations in the vicinity of Steamboat Springs. 16 r:~~~~}~.;:~;-;;i