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(33] reported that rainbow trout were more sensitive to the effects of lead when exposed as eggs, and this could seriously affect the ability of rainbow trout populations to survive in the river. However. since most lead is probably precipitated in natural waters due to the presence of carbonates, Merlini and Pozzi f 1977) 1110] concluded that most lead is unavailable for accumulation by aquatic animals. This can be seen by concentrations of lead detected in brown trout livers throughout the study area. Mean levels were always below con- centrations which would indicate conditions of unacceptable lead exposure. Molybdenum was not detected in any of the river samples and thus would probably not be important in toxicity evaluation even though it is capable of bioaccumulation in aquatic organisms. Cadmium is extremely toxic to aquatic organ- isms and was found in potentially toxic concen- trations throughout the river. The Leadville Drainage Tunnel and California Gulch were the major sources of cadmium pollution. During the high flows of spring runoff, cadmium appeared to enter the river downstream of the Lake Fork inflow or was flushed downstream from sedi- ment scouring caused by high flows. Benoit et al. (1976) 191 showed that cadmium was not eliminated quickly from brook trout livers, and cadmium concentrations in fish livers taken from throughout the study area were always far below levels that indicates Chronically high ex- posure. Heavy metals concentrations in the river are usually higher below a pollution source and lower after a freshening inflow. Higher flows in the spring do not always show these consistent trends due to the suspension of metal-laden sediment. Aquatic Macroinvertebrates The average diversity indices (a) for macroinvertebrates found in the study clearly showed that community structure changed below heavy metals pollution sources Ifig. 20 and tables 4 and 5). After the inflows of the Leadville Drainage Tunnel. California Gulch, Iowa Gulch. and various other inflows from the east side of the river. a and abundance were always reduced. indicating poorer water quality. Freshening flows from the west, Tennessee Creek. Lake Fork, Box Creek. and Lake Creek. always resulted in higher 0 and abundance, in- dicating improved water quality conditions. It is difficult to use 0 or abundance to define or characterize water quality, but comparisons can be made between the stations in this study. Equitability Ie) values did not show the consis- tent trends that 0 and abundance did. indicating that too few organisms were present in many of the samples. I I I I II Ephemeroptera are generally considered to be very sensitive to pollution, but this may not be entirely true, since the mayfly Baetis is found in abundance throughout the study area. An ap- parently more sensitive genus, Ephemerel/a, is found in abundance only above the inflow of California Gulch. A trichopteran, Hydropsyche, is found in some abundance throughout the study area, except at AR-3. Warnick and Bell (19691 11591 found Hydropsyche betteni to be quite tolerant of heavy metals. The free-living caddisfly Rhyacophila was most abundant at EF-l, AR.l, and AR-2, indicating either a more suitable habitat for net-spinning forms. or an in- tolerance to heavy metals pollution. Metal con- centrations in this area of the river were usually lower than downstream. below California Gulch. Hart and Fuller 119741 (701 found that net- spinning caddisflies as a group seem to be tolerant of organic loading, but not of toxic pollutants. Benke and Wallace (1980) [81 also concluded that net-spinning caddisfly produc- tion in a mountain stream appears to be limited by the amount of high quality food available in the seston. The case-building caddisfly Brachycentrus was more common and abundant at stations downstream of the California Gulch inflow. Wiggins and MacKay (197B) [166] classify them as inhabitants of warm lotic condi- tions. The lower reaches of the river were always much warmer than the headwaters. Brachycentrus are also probably more tolerant of heavy metals pollution than are free-living forms. The stoneflies AI/operla and Isoperla were collected at all stations, indicating a relative tolerance to heavy metals pollution. Most diptera larvae have a wide range of en- vironmental requirements and are found in a variety of habitats. Their tolerance to organic and heavy metals pollution is usually quite high, 38