Montana Field Guide

This ecosystem is found throughout the drainages of the Northwestern Great Plains Ecoregion of Montana. Habitats are small to medium (3rd-4th order, <100 river miles long, average wetted width = 5m) perennial warm-water streams, or upstream reaches above Medium Prairie Rivers. In the low elevation (750-1000m) areas, these are low gradient, meandering streams with a typical stream morphology consisting of long runs and continuous standing pools (1-1.5 m depth. Moderate gradient sections may contain infrequently spaced riffles that sometimes maintain wetted connectivity throughout the year. Riffle habitats may be absent in incised and degraded channel sections. Substrate characteristics are typically cobble/pebble riffles (when present) to silted runs and deeply silted pools. The C005c is a prairie stream system within 5 river miles from a confluence with a larger river (at least 2 stream orders larger) and contains a biological fish community that is significantly affected by the large river connectivity.

The diagnostic resident fish community is dominated by the Core Prairie Stream Assemblage and occasionally members of the Medium Warmwater and Creek Chub Assemblages (especially near the confluence areas with a medium praire stream, C005c). A fairly clear Great Plains stream in far eastern Montana with weedy pools could contain brook stickleback, northern redbelly dace or brassy minnow, but for the typical turbid Great Plains stream, the community indicator species include fathead minnow, lake chub, flathead chub, white sucker, creek chub, and introduced species like black bullhead, carp and green sunfish. If the stream has gravel substrate in the riffle areas there will likely be longnose dace, and if there are large cobbles or large woody debris, stonecat as well. A severely impaired C005 community will be dominated by green sunfish and other members of the Sunfish Assemblage including plains killifish, black bullhead and fathead minnow. The Macroinvertebrate Community consists of members of the Large Prairie River and Prairie Stream Assemblages in the riffle/run habitats with the Large River Slow Current Assemblage in the slow current areas, side-channels and vegetated pools. The community indicator species are characterized by the crustaceans (Hyalella and Gammarus), damselfly genera (Coenagrion/Enallagma spp. and Enallagma civile), many genera and species of the water boatman (Corixidae: Sigara grosslineata, Trichocorixa and Corisella), snails (Physella, Gyraulus, and Stagnicola) and mayflies (Caenis and Callibaetis); and caddisflies (Hydropsyche morosa group, and Cheumatopsyche), and riffle beetles (Dubiraphia and Microcylloepus) in the cobble riffles. The giant floater mussel (Pyganodon grandis) is common in the small Northern Glaciated Ecoregion streams, but is rarely encountered in the Powder River Basin streams.

The Great Plains Prairie Stream type occurs throughout the Great Plains region of North America within the Missouri River Drainage. In the Northwestern Great Plains of Montana, we have a diversity of this type including the Otter, Sarpy, Armells, Beauvais, Big and Little Porcupine, Pumpkin, Cabin, Cedar, Sweeny, Sandstone, Glendive and Hanging Woman Creeks.

Linear

Small dams, water diversions, stock ponds and introduced gamefish species have had the most significant negative impact on this community (Winston et al. 1991). Other threats include heavy cattle intrusions to the riparian areas, which causes bank erosion and subsequent sedimentation and siltation. Anywhere dams occur, even small stock pond earthen dams, the downstream reaches are affected by altered water temperatures, unnatural water level fluctuations, and changes in sediment and nutrient transport.