Medium Southwest Montana Basin Intermountain Transitional River
Provisional State Rank
(see reason below)
State Rank Reason
Although widespread and with many representative river reaches in the state, this ecosystem is heavily affected by small dams, water diversions, stock ponds, and introduced gamefish species that have had significant negative impacts on this community. Therefore, biologically intact river miles of this ecological system are rare.
This well-known cool to cold-water ecosystem occurs throughout the upper Missouri and Yellowstone River Drainages and includes some of the most famous trout rivers in the country. Habitats occur in moderate elevation (1200-2000m), medium-sized streams (4th-5th order, wetted width from 15 to 30m, average summer temperature <20°C) with 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 high-elevation steep-gradient mountain streams to the eastern prairie rivers. They are typically direct tributaries of the Missouri and Yellowstone Rivers except the Mainstem Transitional subtype, which describes portions of the Yellowstone and Missouri River. These are classic 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, except during spring run-off or where cattle intrusions, bank erosion, or stream incisement has occurred in the watershed.
The members of the fish community are dominated by the Coolwater Transitional and the Traditional Trout Stream Assemblages. The fish community indicator species were historically dominated by native species, including the Westslope cutthroat trout, mountain whitefish, mottled sculpin, longnose sucker, longnose dace, with the Yellowstone cutthroat trout & mountain sucker occurring 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 Big Hole Assemblage (SPA #6) is a unique assemblage found only in the B001- Big Hole River drainages and includes the indicator species, fluvial arctic grayling (candidate for federal threatened status), and the redside shiner. 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 populations may be in decline due to loss of their native host fish the Westslope Cutthroat Trout.
The Intermountain Transitional River Ecosystem has been identified in the rivers that dominate the trout fishing scene throughout central and southwest Montana; B001-Medium Intermontane rivers, origins in Intermontane basins within the Southwest Montana Basin and Valley including the Big Hole, Beaverhead, Madison, Jefferson, Red Rock Rivers. Additional Intermountain Transitional Rivers include the Smith, Gallatin, Dearborn, Sun and the mainstem Missouri from Three Forks to Cascade. In the Yellowstone drainage, the mainstem Yellowstone River from Gardiner to Big Timber, Clarks Fork of the Yellowstone, the lower Boulder, Stillwater, and Shields Rivers. The Middle Missouri/Musselshell Drainages contain the representatives: Judith and Musselshell Rivers.
Large dams and reservoirs have had the most significant negative impact on this ecological 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 Milltown and above Noxon Reservior 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.