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Runoff from thunderstorms can have a large effect on suspended-sediment concentrations in the lower basin. Two sampled storm events at Catlin Dam illustrate the potential for transport of large sediment loads from tributary drainages to the lower Arkansas River. In July 1991, a fast-moving thunderstorm that was localized in the Apishapa River drainage produced suspended-sediment concentrations in excess of 20,000 mg/L in the Apishapa River (pI. I). Downstream at Catlin Dam, a sharp increase in streamflow and suspended sediment was observed as flow from the Apishapa River reached the site. A comparison of suspended-sediment concentrations and loads at Catlin Dam during the storm runoff and during a similar streamflow in June 1991 (peak flow) showed large differences; concentrations and loads were nearly five times larger from storm runoff than those observed during the peak flow in June (table 28). In August 1992, concentrations of suspended sediment at Catlin Dam following a storm event were about 13 times greater than concentrations measured at a similar flow one month earlier (table 28). Bacteria Water samples were collected and analyzed for fecal-coliform bacteria, total-coliform bacteria, and fecal-streptococcal bacteria at selected sites in the Arkansas River Basin. Twenty-one to 24 samples were collected for fecal-coliform and fecal-streptococcal bacteria at 15 main-stem sites and I tributary site throughout the basin. Sixteen to 22 samples were collected for total-coliform bacteria at 8 main-stem sites in the upper basin only. Coliform organisms, while relatively harmless themselves, are usually present in water that contains enteric pathogens and, thus, can serve as an indicator. Fecal-coliform analysis is a more definitive test for recent fecal contamination, and fecal coliform is the standard organism used for testing sewage, untreated public water supplies, and primary contact waters. Fecal streptococci are used as indicators of contamination of water because the normal habitat of these organisms is the intestines of man and animals. Fecal-streptococci data verify fecal pollution and may provide additional information concerning the probable origin of the pollution (Britton and Greeson, 1987). Concentrations of fecal coliform in the Arkansas River generally increased downstream (fig. 31) and were relatively low in comparison to stream water-quality standards. Median concentrations offecal coliform were less than the biological standard for all sampled main-stem sites. Typically, the stan- dard is 2,000 colonies/l 00 mL but can be as low as 200 colonies/! 00 mL (Colorado Department of Health, 1994). The highest median concentration occurred at Las Animas (162 colonies/! 00 mL). Median fecal-coliform concentrations tended to be highest during snowmelt runoff and post-snowmelt runoff (fig. 31). Median fecal-coliform concentrations were larger in the lower basin, particularly during the post-snowmelt runotT flow regime. In general, fecal- streptococci concentrations also increased down- stream. Median concentrations ranged from 9.5 colo- nies/!OO mL at Leadville to 612 colonies/I 00 mL at Table 28, Comparison of streamflow and suspended-sediment concentrations and loads lor storms and other streamflow regimes at Catlin Dam Streamflow Streamflow, Concentration, Instantaneous Date characterization In cubic leet In milligrams load, per second per liter In tons per day July 2, 1991 June II. 1991 Snowmelt runoff 1,850 959 4,790 July 2, 1991 Stann 1,750 4,770 22,500 Percent difference -5 397 370 August 18. 1991 July 17, 1992 Post-snowmelt runoff 581 724 1,140 August 18, 1992 Stann 613 9,469 15,700 Percent difference 5.5 1,208 1,277 64 Water-Quality Assessment 01 the Arkansas River Basin, Southeastern Colorado, 199G-e3