Montana Field Guide

This ecosystem is found widely throughout the broad Great Plains region of Montana, including many occurrences in the Northern Glaciated and the Northwestern Great Plains Ecoregions. Representative rivers of this ecosystem are usually direct tributaries to the Missouri and Yellowstone rivers, but can have primary connectivity to other Large Prairie Rivers, such as the Little Powder River to the Powder River or Battle Creek to the Milk River. These are larger (4th and 5th order rivers, >100 river miles long, average wetted width of 15m), perennial, warmwater streams with unconfined valley bottom rivers, but are considered "wadeable" in most reaches by the summer months. In the low to mid-elevation (750-1200m) channels, low gradient streams contain long runs and continuous pools (1-1.5 m in depth), whereas moderate gradient sections contain frequently interspaced riffles that maintain connectivity throughout the year, although riffles may be absent in incised and degraded channel sections. Substrate characteristics are typically cobble/pebble riffles (when present) to gravel dominated runs and silt dominated pools. Large woody debris, deep pools, and undercut banks in the lower reaches of these rivers provide substantial fish habitat. During spring and early summer, lower sections of these rivers (B005c-confluence areas) offer spawning and nursery habitat for sauger, walleye, channel catfish, and other Large Warmwater Assemblage fishes.

The members of the resident fish community consist of species from the Large, Medium Warmwater, Sunfish, Creek Chub, and Core Prairie Stream Assemblages. The diagnostic community species include fewer species of the Large River assemblage, except at confluence areas (B005c & B006c), and are characterized predominately by the native minnow and sucker species of the Medium Warmwater Assemblage: fathead minnows, lake chub, flathead chub, plains silvery minnow, western silvery minnow, white sucker, and shorthead redhorse. In the deeper runs and pools, river carpsucker and channel catfish occur, as well as the introduced species, walleye, northern pike, black bullhead, carp, and green sunfish. The riffle areas are inhabited by longnose dace, flathead chub and, where large cobbles occur, stonecat. The medium prairie rivers of the Northern Glaciated ecoregion (B006) are more likely to contain introduced northern pike, black bullhead and yellow perch, while the Northwestern Great Plains rivers (B005) will more likely have introduced green sunfish, crappie, yellow bullhead and rarely, smallmouth and rock bass. Macroinvertebrate Community consists of members of the Large Prairie River, Prairie Stream, and Filtering Collector Assemblages in the riffles, as well as Prairie Stream, Large Prairie River Slow Current and Medium Prairie River Side-Channel Assemblages in run and pool habitats. Community indicator species are characterized by main channel riverine dragonfly species (Ophiogomphus), damselfly genera (Calopteryx and Hetaerina), mayflies (Leucrocuta, Stenonema terminatum, Isonychia, Fallceon quilleri, Ephoron album, Tricorythodes and Caenis latipennis), caddisflies (Hydropsyche morosa group, Cheumatopsyche and Polycentropus), numerous Corixidae, and mussels - fatmucket (Lampsilus siliquiodea) and the side-channel mussel, giant floater (Pyganodon grandis).

The Medium Prairie River type occurs throughout the Great Plains region of North America within the Missouri River Drainages. In the Montana Glaciated Plains Ecoregion (AES B006), examples include Redwater River, Frenchman, Rock, Battle, Lodge, Poplar, Wolf, Big Muddy, and Beaver Creeks. In the Northwestern Great Plains (AES B005), examples include the lower reaches of the Little Powder, O’ Fallon, Mizpah, Pumpkin, Rosebud, Little Beaver, and Beaver Creeks.

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Fluvial processes play a key role in the dynamics of Great Plains rivers and streams. The nature of these processes is often indicated by channel morphology. Meandering channels generally have a shallow gradient, low flow variability, and sediment loads dominated by silt and finer particles, while braided channels are characterized by a moderately steep gradient, high flow variability, and sediment loads dominated by sand and coarser particles (Friedman, 2002). Flooding is the key ecosystem process whereby establishment sites for riparian vegetation are created, seeds are dispersed, and vegetative succession is controlled. However, since Euro-American settlement, natural fluvial processes have been disrupted in many of these systems by dams and diversions. Disruptions include fire suppression, decreased flows and increased siltation rates due to agricultural activities, and introduction of non-native fish species. Native grazers have been largely replaced by domestic cattle. Consequently, there has been a direct loss of woody plant diversity. Furthermore, both channel incision and channel widening have altered flooding regimes, leading to establishment of flood-intolerant species in many areas.

Small dams, water diversions, stock ponds, and introduced gamefish species have had the most significant negative impact on this community (Winston et al. 1991). Anywhere dams occur, the downstream reaches are affected by altered water temperatures, introduced fish, unnatural water level fluctuations, and changes in sediment and nutrient transport.