Benthic Invertebrates of a Large, Sandy River System: The Green and Colorado Rivers of Canyonlands National Park, Utah

From July, 1994 to November, 1995 we sampled the benthos of the Green and Colorado Rivers of Canyonlands National Park, Utah at the request of the United States National Park Service. This study was conceived with a goal of establishing baseline densities of invertebrates in these rivers in preparation for a long-term biomonitoring program. Four remote sites were sampled on each river during each of seven trips. At each site, benthic organisms retained by a 63-mu m mesh were collected from three habitats: sand runs - regions of strong downstream flow and a coarse, shifting substrate; sand beaches - depositional regions with slower currents and finer substrates; and backwaters - cut-off channels with no downstream flow, typically with the finest substrate and no current. Densities of meiofauna and macroinvertebrates were estimated. Three taxa dominated the community: Nematoda, Rotifera, and Oligochaeta. Ostracods and cladocera were common under certain conditions. We identified 28 genera of chironomids and 29 species from 11 families of rotifers. Analysis of variance indicated that total invertebrate densities were different in the three sampled habitats and in the two rivers at different discharge levels. Eighteen separate analyses of variance were employed to test whether the densities of 18 taxa differed significantly between the two rivers, among three habitats, and at different times and discharge levels. Nine taxa exhibited significant differences in density for interactions involving the rivers. Densities of 13 taxa were significantly different between the three habitats or in some interaction involving these habitats. The effects of discharge and/or habitat age were significant for 11 of the taxa. However, these two variables were often significant in the same analyses and in high-order interactions, suggesting that complex dynamics influence the distribution of these organisms