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stratum for sampling. Samples were collected from a single reach in the Gunnison River each year, and coincided with an ongoing study being conducted by Milhouse and Johnson. Samples were collected in a riffle and run habitat type within each reach. Replicate physical and biological parameters were measured within each reach. In addition to sampling within reaches in each stratum, a synoptic survey for turbidity, light transmission and nutrients was conducted prior to the onset of sampling the strata during 1994 and 1995. These surveys were conducted at numerous locations along the entire length of the Colorado River study area. The data was used to help identify areas of nutrient outfall (e.g. sewage effluent), and to help in establishing patterns of nutrient concentrations, turbidity and light penetration along the length of the river corresponding to the stratum being sampled. The survey was conducted prior to each sample run during a stable flow period with minimal localized runoff. Sampling Schedule Work was conducted over two field seasons, 1994 and 1995. Sampling primary and secondary trophic level parameters for this study coincided with companion sampling for fish (see Osmundson 1999). Sampling occurred twice per year, in March prior to upper basin runoff and in September after the summer thunderstorm season. These times were selected because they represent two distinct seasons and occur during periods of relative system stability. Table 1. Physical and biological parameters collected within each sample reach for selected run and riffle. WITHIN SAMPLE LOCATION UNITS WITHIN REACH UNITS Physical Physical water depth meters nutrients suspended or dissolved mg/L in water column bottom and average column ft /sec turbidity NTU water velocity dominant substrate size D16, D50, light extinction k D84 interstitial substrate size mm habitat distribution m2/reach fractions depth to embeddedness cm drifting coarse particulate gm/m3 organic matter percent surface area % embedded interstitial void volume cm 3/M2 Biological Biological periphyton biomass mg/m2 biomass of macroinvertebrate gm/m3 drift macroinvertebrate biomass gm/mz coarse particulate organic gm/m2 matter 5