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0091 Dissolved Oxygen When considerable oxygen demand is present in a stream such as Fountain Creek, dissolved oxygen may be substantially depleted while digestive proces- ses are occurring. However, the dissolved-oxygen concentrations, which were measured in Fountain Creek during the daylight hours and, therefore, are likely to be somewhat larger than the minimum daily dissolved-oxygen concen- tration, indicate that the stream generally is well oxygenated (fig, 16), Since 1975, only four dissolved-oxygen measurements were less than the recom- mended standard of 5,0 mg/L for Lower Fountain Creek. The stream remains well oxygenated probably because of large reaeration rates that result from large velocities and shallow depths in Fountain Creek, Monthly variations in dissolved-oxygen concentrations of Fountain Creek at sites F12, F17, and F36 are summarized in figure 17, Because dissolved- oxygen concentration is inversely related to water temperature, the smallest dissolved-oxygen concentrations of Fountain Creek occur during July and August when water temperatures are warmest and biological decomposition is greatest (fig, 11); the largest dissolved-oxygen concentrations occur during December and January when water temperatures are coldest and biological decomposition is least, Suspended Solids The suspended-solids concentrations at nine sites along Fountain Creek from 1975 through 1983 are summarized in figure l8A, and suspended-solids concentrations from 1980 through 1983 are summarized in figure l8B, Down- stream variations in suspended-solids concentrations are better represented in figure l8B because the period of record and the number of measurements are more similar between sites. The suspended-solids concentrations measured on Fountain Creek upstream from the confluence with Monument Creek have been small because of the pres- ence of nonerosive igneous and metamorphic rocks, the cobbled-bed channel, and the shallow soils in the Upper Fountain Creek basin, The small concen- trations of suspended solids result in a clear stream, which adds to the aesthetic quality, However, suspended-solids concentrations rapidly increase downstream from the confluence with Monument Creek primarily as the result of Monument Creek inflow, as indicated by data collected at site F17, Suspended- solids concentrations decreased from site F17 to site FI9 because of dilution from the Colorado Springs Wastewater Treatement Plant effluent, The median suspended-solids concentration (1980-83) decreased from 156 mg/L at site F17 to 89 mg/L at site FI9 (fig. l8B), which is located about 2 mi downstream from the wastewater-treatment plant outfall. However, the resulting increase in streamflow from the effluent re-entrains sediments that have been deposited in the stream channel downstream as the result of tributary inflows, which increases the suspended-solids concentrations downstream from site F20 (fig, l8B) , 29