Northwestern Glaciated Large Prairie River
Provisional State Rank
(see reason below)
State Rank Reason
Diversion, flood control, and hydroelectric dams have significantly altered large portions of the Large Prairie River Ecological systems in Montana. The Powder River is the only undammed large prairie river remaining in the state. Many of the characteristic invertebrates of a properly functioning large prairie system are Globally Rare and only occur in a few river reaches in the state. At least one fish in this community is a Montana Species of Concern, the sturgeon chub (S2). It also contains the globally rare sand-dwelling mayfly group, whose members are currently ranked S1-S3 in Montana. The occurrence of numerous threatened, rare and declining species, and consistent ongoing threats of diversions and oil and gas production to the habitats required for spawning and rearing warrant a state rank of S3.
This ecosystem consists of mainstem prairie tributaries to the Missouri and Yellowstone Rivers: the Milk, Marias, Little Missouri, Lower Tongue, Bighorn and Powder Rivers. These large (5th order and larger, >200 river miles long, 35m average wetted width) warm-water rivers have low to moderate gradients. These low elevation (below 1000m) rivers are characterized by long deep runs, pools (1-2m deep), and interspaced riffles. Substrate characteristics are typically cobble riffles (when present) to sand and gravel dominated runs and pools, with variably textured side channels. Large woody debris and undercut banks in the lower parts of these rivers provide substantial fish habitat. During spring and early summer, lower sections of these rivers offer many miles of spawning/nursery habitat for sauger, walleye, channel catfish, and the characteristic large fishes of the Yellowstone and Missouri: pallid sturgeon, shovelnose sturgeon, and blue sucker. The A003c is a prairie river system within 20 river miles from a confluence with a larger river (at least 2 stream orders) and contains a biological fish community that is significantly affected by large river connectivity.
Fish Community: The members of this community consist of the Large, Medium Warmwater River and the Creek Chub Assemblage. The indicator species are characterized by the native species: channel catfish, stonecat, mooneye, sauger, flathead chub, plains minnow, sand shiner, white sucker, shorthead redhorse and some introduced species, including the walleye, northern pike, black bullhead, and the spottail shiner (Milk & Marias Rivers). The shallow riffle habitats are inhabited by the longnose sucker, longnose dace and flathead chub with the mountain sucker included in the Yellowstone drainages. The state threatened sturgeon chub has good populations in the gravel run habitats of the Powder River, but not in any other A003 or A004 river of MT. Macroinvertebrate Community: This community consists of members of the Transitional Prairie River, Large Prairie River and Filtering Collector Assemblage in the riffles, with Large River Slow Current and Medium River Side-Channel Assemblages in the slow current areas and side channels, and the uncommon sand-dwelling mayfly community group (SDM) in the vast sandbar areas. The indicator species are characterized by main channel riverine dragonfly species, Stylurus and Ophiogomphus , the mayflies- Neochoroterpes oklahoma, Choroterpes, Camelobatidius, Fallceon quilleri, Acentrella insignificans, Ephoron album, and Travarella albertana; caddisflies ( Leucotrichia pictipes, Neotrichia, Psychomyia, Hydropsyche morosa group and Cheumatopsyche), and the unionid mussels - fatmucket ( Lampsilus siliquiodea), black sandshell (Ligumia recta) (Missouri main stem only), and the side-channel mussel, giant floater ( Pyganodon grandis ). Although not reported in the classification due to rarity in the samples, the unique sand-dwelling mayfly assemblage includes indicator species: Analetris eximia, Raptoheptegenia cruentata, Lachlania saskatchewanensis, Anepeorus rusticus, Ametropus neavei and Homoeoneuria alleni , and is most closely associated with the Large Valley and Prairie River Assemblage.
Large Prairie River Ecosystem types occur throughout the Great Plains region of North America within the Missouri River Drainage, with notable rivers such as the Niobrara and Platte (Nebraska), Kansas River (KS), Belle Fourche and James Rivers (ND & SD). The Montana Glaciated Plains subsection includes the lower Milk and Marias River watersheds below Fresno and Tiber Dams, respectively, to their confluence with the Missouri River. Within the Northwestern Great Plains subsection, Montana has the Lower Bighorn River from Hardin to the Yellowstone, the Powder River from the Wyoming border to the Yellowstone and a 100-mile stretch of the Little Missouri from the Wyoming to the ND border. This section of the Little Missouri is more typical of a medium-sized prairie stream, but falls in to the Large Prairie River type further downstream in ND.
Riparian cottonwood stands as well as the flood plain dynamics that enhance recruitment of willows and cottonwood saplings must be maintained. Russian olive and salt cedar are widespread introduced species that have reached nuisance levels along some stretches of the Yellowstone, mostly from Laurel downstream.
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. Modifications have included fire suppression, decreased flows and increased siltation rates due to agricultural activities, and the 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.
Diversion, flood control, and hydroelectric dams, bank armoring, and irrigation withdrawals have had the most significant negative impact on this community. Diversion dams create barriers to the long distance spawning runs many fish in this community need, and reservoirs have submerged considerable spawning habitat. Inter-dam reaches below Yellowtail, Marias, Tongue River, and Tiber Dams maintain some of their pre-development channel morphology, but they are affected by altered water temperatures, unnatural water level fluctuations, and changes in sediment and nutrient transport.