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gauging station at Somerset is above these diversions, and • thus, lower summer stream flows occur downstream than those recorded at the USGS station. When flow was estimated at 175 cfs near the Mt. Gunnison No. 1 Mine study area, less than five cfs of flow was present at Paonia. At this flow the stream was limited to small interconnected pools. The substrate of the North Fork of the Gunnison River is predominantly large rubble and boulders. Large boulders were more common at the upper station. At the lower station the stream divides into two channels. The north channel is artifically maintained as a series of pools to function as a pre-settling basin for the intake to the Somerset filtration plant. The south channel is a fast water region with a substrate of small rubble. Chemical Parameters North Fork of the Gunnison River The major cations are calcium, magnesium, sodium, and potassium. The major anions are sulfate, chloride, and carbonates. These ions comprise nearly all of the dissolved inorganic material in natural waters. If the concentrations • of all the ions have been precisely determined, the total milli-equivalents per liter of anions should exactly equal the total milli-equivalents of cations. The difference in the height of the bars for the cations and anions in these figures is probably the result of analytical error. One of the major factors that determines the chemical composition of a stream is the type of rock that underlies the drainage basin. The headwaters of the streams occurring on the Mt. Gunnison No. 1 Mine study area are largely underlain by igneous and associated metamorphic rocks that are relatively resistant to dissolution by water. The che- mical composition and dissolved solids concentration recorded from the streams are typical of an area underlain by igneous rocks; however, sedimentary rocks which form the streambeds in lower reaches increase dissolved solids and alter chemical composition of the waters. Two trends in the chemical composition and concentra- tion of the major chemical constituents in the drainages on the study area were evident, a seasonal trend and a down- stream increase of dissolved solids concentrations. The North Fork of the Gunnison River drains a much larger and more diverse area than the other drainages. • However, the similarity in geology of the upper areas 2.04-68