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benthic insects was highest for gravel with a mean diameter between 2.5 to 3.5 centimeters. It was believed that this size of substrate was better to collect detritus that was used as food by the grazers and scrapers than smaller gravel (0.5 to 2.0 cm) or larger gravel (4.5 to 7.0 cm). The interspaces between the 2.5 to 3.5 cm gravel collected small detritus (primarily leaf fragments and soft bark less than 4.0 mm in size) used as food by detritivores. Water velocity decreased rapidly in the smaller gravel and was too great in the larger gravel to accumulate small detritus. Predatory insects colonized the trays in relation to the density of Rrey. Normal flow used as the control had a mean water velocity of 48 cm S'l. The numbers of organisms was less in six out of sixteen taxa that were studied when the flow was reduced to a mean water velocity of 11 cm S.l. Si lt accumul ated in the substrate at the lower water vel oc it i es. The substratum-detri tus interact i on was cons i dered most important in the microdistribution of benthic insects. Water velocity or a light deposition of silt were considered to be secondary. The experimental results of Rabeni and Minshall (1976) are supported by the literature of benthos in running waters (Hynes 1970; Welcomme 1985) that document benthic macroinvertebrates in riffles with large cobble substrates produce a higher diversity of species and larger numbers of organisms than river reaches composed of sediments of sand and silt. Benthic invertebrates from productive floodplain waters move or are flushed into the main channel, thereby providing additional food to fishes in the main channel (Eckblad et al. 1984). Obviously, different species of insects vary in their requirements and their tolerance to these factors. However, inferences can be made to benthic insect abundance and distribution in the Upper Colorado River Basin. Riffle areas with large gravel substrate will be more productive than pools with sand substrate. The low velocity habitats used by the larvae and juveniles of the endangered fishes contain sand and silt substrates that would be expected to have a low number of benthic invertebrate taxa. Chironomids are the most abundant benthic invertebrates in these substrates that are eaten by large larvae and juveniles of all species of endangered Colorado River fishes. This taxon would be the best indicator of benthic insect production as food for juvenile endangered Colorado River fishes. VII. BENTHIC PRODUCTION IN DIFFERENT HABITATS OF THE MIDDLE GREEN RIVER Since chironomids constitute a significant part of the diets in juvenile endangered Colorado River fishes (Bestgen 1990; Grabowski and Hiebert 1989; U.S. Fish and Wildlife Service 1990a, 1990b, 1991), this discussion will emphasize that taxon in the benthic production of the upper basin. The juvenile endangered fishes inhabit low velocity habitats such as embayments and backwaters along the main channel where the substrate is composed of silt, detritus, and sand. This is the type of substrate that is generally inhabited by chironomids. The numbers of chironomids, found in the main channel of the middle Green River, ranged between 360 to 4,645 organisms per square meter and the numbers ranged from 4,820 to 28,860 organisms per square meter for backwater habitats in 1987 (Table 1; Grabowski and Hiebert 1989). In 1988, Grabowski and Hiebert reported the range in numbers of chironomids between 280 and 5,000 organisms per square meter for the main channel of the river and between 3,330 and 28,890 organisms per square 7