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• Pennakl 1973) has listed r ( anges of densities found in Rocky Mountain streams in Colorado, Utah, Wyoming, Idaho, and Montana. These range from approximately 50 to 1800 organisms per m 2. In Colorado, ranges xere from approximately 200- 1,000 with associated biomass of approximately 1.6 - 12 g/m2. The Crystal River, which is a drainage ,lust over McClure 's Pass from the North Fork,had an average density of 188 (range approximately 50-600) from 4 stations sampled 5 times over a May-September period in 1972. Corresponding biomass ranged from 0.4 - 17.6 g/r.2. By comparison, densities found at the North Fork (averages of 2-3 samples/station) ranged from 700- 14,000 when chironomids and ceratopogonids were included, or approximately 300- 5,000 per m2 when they were not*. This indicates relatively high densities of organisms in the North Fork. Biomass estimates were only available for the March & July samplings ir. the • present study. In March these values ranged ~'imarily from 4-13 g/m2 with one * outstandingly high value of 70 g/m In July-, corresponding to the drop in numbers of organisms, the biomass estimates were lower, ranging from 0.3 - 6.2 g/m2. Thus biomass estimates for the North Fork, were within the range of those Found for the Crystal River and other Colorado mountain streams (xith the exception of the 70 g/m2 value). However the overall average biomass/m2 x2s 2-4 times higher for the North Fork than for the Crystal (2.4 g/m2for Crystal vs 5.5 & 9.8 g/m2for the North Fork, without or with the 70 g value.) * wo^}: or. the Crystal River did not indicate many chironomids and ceratopogor.ids This may have been a function of sampling technique. • this latter figure was principally due to very high levels of caddisflies (which included cases) and snails. These two groups contributed 760 ~ 8o to the 70 g/mz total. ~ ~ Thorne, '-973 =.. 5-7