Missouri Mainstem Intermountain Transitional River
Provisional State Rank
(see reason below)
State Rank Reason
A viable, fully functioning Mainstem Intermountain Transitional River is uncommon in the state. Diversion, flood control, and hydroelectric dams have significantly altered large portions of this Large River Ecosystem in Montana. Many of the native fish species that inhabited this river reach have been replaced by introduced salmonids (Rainbow and Brown Trout) and other predator fish (northern pike, walleye and smallmouth bass).
This cool to cold-water transisitonal ecosystem occurs throughout the upper Missouri and Yellowstone River Drainages and includes some of the most famous trout fishing river sections in the state. Stream habitat occurs at moderate elevation (1200-1600m), medium-large sized streams (6th & 7th order, wetted width from 25 to 50m), average summer temperature ~20°C) with low to moderate gradient and a permanent flow. There is strong seasonal variability due to melting snow pack from higher elevation mountainous areas. These rivers represent the ecotonal area from mid-elevation and moderate gradient intermountain streams to the eastern prairie rivers. There are reaches of the Missouri and Yellowstone Rivers that are marginal freestone trout rivers with boulder/cobble riffles, cobble/gravel runs and pools, and silt on the stream margins or in the deepest pools. Deep runs and pools with undercut banks and large woody debris provide the best fish habitats, while the riffles harbor the most diverse macroinvertebrate communities. These are clear
running rivers reaches, except during spring run-off or where cattle intrusions, bank erosion or stream incisement has occurred in the watersheds.
The members of the fish community are dominated by the Coolwater Transitional and the Traditional Trout Stream Assemblages. The fish community indicator species would have typically been dominated by native species: the Westslope cutthroat, mountain whitefish, mottled sculpin, longnose sucker, longnose dace, with the Yellowstone cutthroat trout & mountain sucker in the Yellowstone drainages. However, the introduced species of the stocked trout assemblage, the brown and rainbow trout, tend to dominate and become the focal species of these systems. One member of the Large Mainstem Warmwater River Assemblage (#5) that occurs frequently in deeper, coldwater habitats of this system is the burbot, a potential SOC in the state. Additionally, the white sucker, walleye, northern pike and the exotic carp may be found at the warmer, lower end of this transitional gradient. The shallow gravel runs of these rivers provide spawning habitat for downstream populations of brown trout during their fall migration, and rainbow trout and sucker species in the spring. The diverse macroinvertebrate community consists of members of the Medium Cool-Water Transitional Assemblage, the Traditional Trout Stream assemblage, Medium Mountain Stream Community and the Foothills Transitional Assemblage. The community indicator species are characterized by main channel, fast current stonefly and caddisfly species: Pteronarcys californica, Hesperoperla pacifica, Brachycentrus americanus, Arctopsyche grandis, Hydropsyche, Glossosoma, Lepidostoma and the tipulids: Hexatoma and Antocha . Mayflies are diverse in this system and contain many genera, including: Baetis, Ephemerella, Serratella, Rhithrogena, Drunella and Epeorus . As these transitional rivers proceed downstream and begin to warm (>17 C) or are sediment-impaired, degraded, or dewatered, they will quickly lose the Traditional Trout Stream and Medium Mountain Stream Community (#4 and 90), and shift to the mayfly, caddisfly, beetle and dipteran species that form the Medium Cool-Water Transitional Assemblage (#1) and the Foothills Transitional Assemblage (#105), with indicator species: Hydropsyche, Optioservus, Baetis tricaudatus, Brachycentrus occidentalis, Helicopsyche borealis, Prosimulium, Amiocentrus aspilis, Lara, Plauditus, Narpus. Populations of the western pearlshell mussel have been reported from this river ecosystem, although the many of these populations may be in decline.
The Large Intermountain Transitional River type occurs in the Missouri River upstream of Great Falls to the Three Forks area, including the Blue Ribbon Missouri River section between Holter and Cascade. It also occurs in the Yellowstone River upstream of Billings to the Livingston area. This system also includes medium-sized, transitional rivers flowing through Central Montana Broad Valleys including the Upper Missouri River, Lower Gallatin, Jefferson, and Madison Rivers.
The environment within which this ecological system is found is typically the broad sage/grassland valleys with cottonwood riparian areas characteristic of the Central Broad Montana Valleys Ecoregion.
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.
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 Reservoir 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.