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beak aliquots to determine (1) generic composition of the periphyton, (2) dry • and ash-free weight, and (3) chlorophyll a content. The generic composition of the periphyton was determined by filtering a suitable aliquot onto a 0.45um pore-size membrane fil*_er, making the filter transparent, then identifying to genus and counting 200 discrete algae particles at a magnification of 400x. Taxonomic identifications were made using the descriptions of: Patrick and Reimer 1966, 1975; Drouet 1959; Geitler 1932; Bourrelly 1968; and Prescott 1962, 1970. The procedure for determining the dry weight and dry weight organic matter (ash-free weight) of the periphyton entailed removing visible insect larvae from the periphyton, filtering the entire quantity of remaining material onto tared GF/C glass fiber filters, drying the filters at 105°C, weighing the filters, then ashing the filters at SOOoC and re-weighing • (APHA 1976). The procedure for determining chlorophyll a, another measure of biomass, entailed filtering periphyton material onto 0.45um pore-size membrane filters, grinding the filters to disrupt algae cells, then recording absorbances on a spectrophotometer with a resolution of Bum (APHA 1976). 4.2 Results and Discussion Attached algae on the stream bottom substrate of the tlorth Fork Gunnison River was sampled at four stations, C, D, E, and F. Generic analysis of the periphyton showed diatoms of the division Chryspophyta were the most numerous genera; blue-green and green algae, of the divisions Cyanophyta and Lhlorophyta, respectively, were represented by only three genera (Table 4.0-1). The most • abundant genus at stations C, D, ad E was the blue-green Schizothrix (Tables 4.0-2, 4.0-3, and 4.0-4) and at station F the diatom genus CYmbella (Table 4.0-5). 4-2