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TABLE 4: Mean biomass (grams/mz -wet weight) of major invertebrate groups from Rito Seco creek, Costilla County, Colorado, 16-17 August 1988. Stations TAXA 1 2 3 ' INSECTA Ephemerop[era Plecoptera 1.7512 0.0320 0.5971 0.9048 0.6420 0.2065 Trichoptera 1.1492 2.5871 3.3034 Diptera 1.5268 1.4759 3.0603 Coleoptera 1.0603 0.2391 0.4849 MOLLUSCA ' Gastropoda -- -- 0.0626 OLIGOCHAETA 0.0147 0.0003 0.0007 1 TURBELLARIA 0.1495 0.0003 -- TOTAL 5.6837 5.8046 7.7604 1 ' creek exhibited densities of 226 and 196 organisms/mz and biomass of 2.57 and 3.07 g/mz (Chadwick & Assoc. 1986). Based on these comparisons the data from Rito Seco creek indicate healthy, productive populations. These comparisons are all from data collected during fall, generally late September and ' October. The general trend ir. Colorado is for peak densities and biomass to occur during spring, prior to runoff and emergence (Chadwick and Canton 1983, 1984). High flows during runoff and the emergence of aquatic, immature insects to the terrestrial, adult stages reduce summer benthic densities significantly. During fall and winter, population levels are increasing from the summer minimum until the spring maximum. Rito Seco creek was sampled during mid-August, usually a period of low densities and biomass. However, the densities and biomass levels are comparable to the fall levels in other streams. The relatively high summer populations can partially be ' explained by the fact that records indicate peak flow through the study section in 1988 peaked at only 9-]0 cfs in early June of 1988 (see Exhibit G). This appears to be a low peak flow for a stream of this size and could have resulted in the unusually high density and biomass levels observed in the summer of 1988. ' Species composition was similar at Sites 1 and 2 with a diverse group of mayflies (Ephemeroptera), stoneflies (Plecoptera) and caddisflies (Trichoptera) present. At Site 3 mayflies ' 10 1