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tolerant of polluted water than the other groups, low values of this ratio are often used as <br />indicators of poor water quality. While the ratios are consistently higher upstream of the <br />discharge in the October data, the standard deviations are also higher. The numbers of <br />Plecoptera and Ephemeroptera, which are also relatively intolerant of poor water quality, do <br />not show a consistent change from upstream of the discharge to downstream. The decrease in <br />the EPT /C ratios downstream of the discharge is primarily a result of the increase in <br />Chironomids, rather than a decrease in the intolerant groups. October ratios were also affected <br />by large numbers of Trichopterans in the two farthest upstream sites (U4 and U5). There was <br />no clear trend in Simpson's diversity index, and in fact, the least diverse site was upstream at <br />U4. <br />Regarding the physical habitat, we see no particular trend in channel morphology, current <br />velocity, or stream bank condition that would significantly influence aquatic macroinvertebrate <br />density. Currents are slightly higher in the downstream stations, and there are two small areas <br />where vertical stream banks indicate active erosion (the largest of which is downstream). In <br />general, the stream banks are relatively stable. Some silt deposition was observed downstream <br />of the discharge hugging the north bank of the river. This was observed at station D2 and <br />elsewhere (though not at any of the other cross - section locations downstream). A possible <br />source is the Sanborn Creek watershed. The presence of silt could impact aquatic insects and <br />cause an increase in Chironomids. The highest Chironomid numbers occurred in the samples <br />collected from station D2. <br />Field measurements of water quality and chemical analysis of water samples indicated that the <br />plume was detectable for several hundred meters downstream. Of the field measurements, <br />electrical conductivity provided the most reliable indicator of the plume. Analysis of water <br />x samples also demonstrated detectable plume effects, although the effects were more <br />pronounced in the fall samples when the river volume seemed to be lower. In the spring <br />samples, no effects were seen from D1 (immediately downstream of the discharge), probably <br />because the sample was collected out of the plume, and for some reason, the sample from U4 <br />exhibited slightly higher values for TDS, electrical conductivity, sodium, chloride, and <br />bicarbonate. TDS, sodium, chloride, bicarbonate, and sulfate values were also slightly <br />elevated in the sample from station D5. Perhaps activities at the West Elk and Bear mines had <br />some effect on these two samples, respectively, though no surface discharges were observed at <br />the time of sampling. We do not know what the background water quality is of Sanborn <br />Creek, and if possible elevated levels of dissolved constituents from that watershed could exert <br />a chronic effect on the aquatic macrofauna, even without the effect of the mine discharge. <br />Because of the surficial geology of the watershed (fine sandstone and shales), it is likely that it <br />would be at least a minor source of dissolved materials as well as fine sediments. <br />Conclusions: <br />1. Numbers of aquatic macroinvertebrates are moderately high both upstream and downstream <br />of the discharge, including intolerant groups. Chironomid numbers increase dramatically <br />(0 21 <br />