Montana Field Guide

This ecosystem is found widely throughout the mainstem Missouri River system (7th order and larger) of the midwestern and western United States. These large, warm-water rivers have low to moderate gradient with origins in the intermontane basins of Montana. Throughout the range, river elevation is below 900m and is characterized by long deep runs and pools with depths >2m, numerous mid-stream islands, side channels and interspaced riffles. Substrate characteristics are typically cobble in the riffles, whereas sand and gravel dominate runs and pools, with gravel and/or finer-textured side channels.

Fish Community: The members of this community consist of the Large, Medium, and Large Mainstem Warmwater River Assemblages. The community indicator species are characterized by main channel species including shovelnose sturgeon, pallid sturgeon, freshwater drum, paddlefish, burbot, sturgeon chub, sicklefin chub, and blue sucker. The Missouri mainstem contains an additional species not found in the Yellowstone River, the shortnose gar, which has only been recorded downstream from Fort Peck dam. Large Valley River fish communities also include the side-channel communities occurring at the margins of the main current or in the quiet side-channels; these include emerald shiner, channel catfish, mooneye, sauger, flathead chub, carp, white sucker, shorthead redhorse, and sand shiner. The shallow riffle habitat areas are inhabited by longnose sucker, longnose dace, and flathead chub with mountain sucker in the Yellowstone River. 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.

In Montana, the Large Valley River type occurs in the Missouri River downstream from Great Falls, especially past the confluence with the Marias River, downstream below Fort Peck Reservoir and on down to the North Dakota border. The Yellowstone River downstream from Billings to the Missouri River Confluence. Additionally, the Lower Powder, Tongue, and Big Horn Rivers have occurrences of the Large Valley River fish assemblage during spring run-off.

Linear

The Large Valley River type occurs in the Missouri River downstream from Great Falls, especially past the confluence with the Marias River, downstream below Fort Peck Reservoir to the North Dakota border and the Yellowstone River downstream from Billings to the Missouri River Confluence. The Yellowstone River transitional area to a characteristic Valley River begins below Billings to around Pompeys Pillar. Additionally, the Lower Powder, Tongue, and Big Horn Rivers have occurrences of the Large Valley River fish assemblage during spring run-off.

Riparian cottonwood stands as well as the floodplain 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. Additional disruptions to the natural dynamics of this system include 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.

Large dams and reservoirs have had the most significant negative impact on this community. 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 Great Falls to Fort Peck Reservoir and between Fort Peck and Lake Sacagawea, ND) 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.