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Substrate samples were taken from each station and sieved into size classes • (Table 2.7-I I). Stations I, 2 and 4 appear to have similar substrate dominated by The gravels (especially coarse and medium gravel). Station 3 was predominately rubble substrate, although the gravel component was still important. It should be noted that the methods used To collect substrate precluding sampling the larger components such as cobbles and boulders. There was no discernable increase in the fines at the downstream sites, despite the low gradient of the stream. A pparenTly the Jorge spring snowmelt runoff flushes any fine sediment that accumulates in the riffles over the year. The substrate was also analyzed with respect to particle size at The stations on Oak Creek (Table 2.7-12). The results show that both stations hove similar substrate types dominated by medium gravel with coarse and fine gravel providing the bulk of the remaining size classes. Coarse gravel was more important at Station I perhaps reflecting the swifter current at this station. Periphyton - For the purposes of this study, periphyton is defined as the algal community found attached To various substrates such as rock surfaces, logs, etc. These are the prim ory producers and the food base for the stream. Although this algae may not be while living, it is consumed by many organisms after it has • died and begins to decompose (Minshall 1978). The algae of swift flowing Rocky Mountain streams like Trout Creek are distinctive in both species composition and life habits (Palmer 1977). Diatoms (Bacillariophyta) predominate in number of species. Since they are small, primarily unicellular and often attached To the substrate, they are not as affected by the turbulence of mountain streams as other algal orders. The other major members of the periphyton are the green algae (Chlorophyta) and blue-green algae (Cyanophyta). The community and its pro- ductivity in the stream changes with season and is dependent upon a variety of factors. Current velocity appears to be very important to the periphyton as does light intensity, with production increasing with increased current and light (Pfeifer and McDiffet 1975). A total of 107 species of algae were collected in Trout Creek. Less than 30% of these were found at all four stations (Appendix 2.7-C - Table I). There were greater numbers of species at Station I and 2. Typical of stony streams in Colorado, diatoms were the dominate member of The periphyton, with blue-green, green and yellow-green algaes of only marginal importance. At Station I diatoms were less • important and blue-greens and yellow-greens more important than at the other stations. Diatoms were more important aT the other three stations, with blue-green 2.7-42