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00072~ 440 CM, Tate and J,S, Heiny and temperature (Vannote & Sweeney, 1980; Ward & Stanford, 1982), particularly in streams extending along altitudinal gradients (Ward, 1986), Other factors affecting invertebrate communities are land use (Quinn & Hickey, 1990) and the characteristics of the riparian zone (Gregory et ai"~ 1991), Altitudinal changes in invertebrate communities along the Saint Vrain River, a tributary of the South Platte River, were described by Ward (1986) and demonstrated natural variability in invertebrate com- munities primarily in mountain areas, although the study also included a plains site, In contrast, data were collected on a basin-wide scale during the present study to determine how invertebrate communities vary spatially in mountains and plains streams affected by natural and human factors, The objectives of this study were to describe the spatial distribution of invertebrate communities within the South Platte River Basin and to relate this distribution to physical and chemical factors within the basin. Materials and methods Description of study area The South Platte River Basin drains a 62 900-km2 area including parts of Colorado, Nebraska and Wyoming, US.A. (Fig_ 1), A detailed description of the basin environmental setting is given in Dennehy et aI, (1993), There' are two major physiographical provinces (Lobeck, 1922), the Southern Rockies (mountain region occupying 25% of basin area) and the Great Plains, which corresponds to the two major ecoregions, South- ern Rockies and Western High Plains (Omernik, 1987), Altitude ranges from 850 to 4370 m, which affects climatic, vegetational and physical settings of the streams in the basin (Dennehy et aI., 1993), Selected physical characteristics and land-use classifications of sites sampled are presented in Table 1. Most tributaries originate in the mountains and are characterized as cold-water streams receiving SO- lDO em of annual precipitation, Stream slopes are steep and narrow channels are cut into resistant crystalline bedrock forming riffle/pool habitats with boulder/ cobble substrates, Streamflow is perennial and affected by snowmelt runoff in the late spring/ summer, From the mountains to the plains, streams cross a transition zone of easily eroded sedimentary bedrock. Streams in the plains are characterized as warm- water, low-gradient streams that receive less than 50 em of annual precipitation, The streams flow through easily eroded and transported alluvium, form- ing mostly run habitats with sand/gravel substrates, Channels are shallow and occasionally braided, Tribu- taries originating in the plains are intermittent with storm events affecting the streamflow, Water management in the South Platte River Basin has considerably altered the natural hydrology_ The quantity of water has increased due to interbasin transfers from the Colorado, Arkansas and North Platte River Basins, Flow within the basin has been modified in space and time through a complex network of ditches and reservoirs primarily for municipal water supplies and agricultural irrigation (Dennehy et aI., 1993)_ The South Plalle River in the eastern part of the basin is now a perennial stream, whereas historically (prior to the 1860s) streamflow was intermittent (Eschner, Hadley & Crowley, 1983), Land-use/land-cover categories for sampling sites were defined by the Anderson classification system (Anderson et ai"~ 1976), Siles in the mountain and transition zones were dominated by forest, built-up (low-density housing and population), and rangeland classes, with one site affected by mining (Clear Creek at Golden, Table 1), Once streams enter the plains region they flow through urban (high-density housing and population) and agricultural land-use areas, or a mixture of both, The mixed agriculture/urban classi- fication was defined as sites located in agricultural areas that were affected by contaminants from upstream urban areas (not an Anderson classification), l I I I Sampling regime Twenty-one sites were selected for sampling by strati- fying the basin streams by physiographical province/ ecoregion, geology, land use/land cover and spatial coverage of the basin (Fig, 1, Table I), Invertebrate samples were collected from 27 July to 7 August 1992 at sites ranging in altitude from 850 to 3132 m (Table 1)_ By sampling only once during the summer (low flow), the total number of taxa collected at a site was lower than the total number of taxa reported by Ward (1986) because of the absence of winter species or early emergence. It was assumed that differences in the patterns of invertebrate communities reflect actual differences among sites and not seasonal trends. @ 1995 Blackwell Science Ltd, Freshwater Biology, 33, 439-454 ,