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Montana Field Guides

Westslope Intermountain Transitional River

Provisional State Rank: S4
(see reason below)

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State Rank Reason
Widespread and with many representative river reaches in the state, this ecosystem has been affected by small dams, water diversions, stock ponds and introduced gamefish species that have had significantly negative impacts on this community. Thus, outside of wilderness areas such as the Bob Marshall, it is fairly rare to find biologically intact river miles of this ecological system.

General Description
This well-known cool to cold-water ecosystem occurs throughout the upper Columbia River Drainages and includes some of the most famous trout and wilderness rivers on the western side of the state. 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 ecotone between high-elevation, moderate gradient mountain streams and intermountain valley rivers. They are generally direct tributaries of the Clark Fork or Flathead Rivers, except for the Mainstem Transitional subtype, which describes portions of the Clark Fork River itself. These are classic freestone trout rivers with boulder/cobble riffles, cobble/gravel runs and pools, and silt on the 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 heavy livestock use, bank erosion or stream incisement has occurred in the watersheds. The (L) descriptor on this AES code depicts the effects of proximity to a lake outlet which can have a significant effect on the biological community.

Diagnostic Characteristics
The members of the fish community are dominated by the Coolwater Transitional and the Traditional Trout Stream Assemblages. The westslope fish community indicator species would typically be dominated by native species such as the Westslope cutthroat, mountain whitefish, Rocky Mountain or slimy sculpin, northern pikieminnow, largescale sucker and longnose dace. These rivers also provide strongholds for the Bull Trout in reaches that have not been impaired anthropogenically. 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 such as Pteronarcys californica, Hesperoperla pacifica, Brachycentrus americanus, Arctopsyche grandis, Hydropsyche, Glossosoma and Lepidostoma ; and by the tipulids Hexatoma and Antocha. Mayflies are diverse and contain many genera, including Baetis, Ephemerella, Serratella, Rhithrogena, Drunella and Epeorus. As these transitional rivers proceed downstream and begin to warm (>17C) or are sediment impaired, degraded or are 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 such as Hydropsyche, Optioservus, Baetis tricaudatus, Brachycentrus occidentalis, Helicopsyche borealis, Corynoneura, Prosimulium, Amiocentrus aspilis, Lara, Phaenopsectra, Plauditus, and Narpus. Populations of the western pearlshell mussel have been reported from this river ecosystem, although the populations may be in decline.

AES B009 Medium Intermontane rivers originate in the Bitterroot Valley or Beaverhead Mountains including the Bitterroot, Clark Fork, Rock Creek, Blackfoot and Clearwater Rivers. AES B010 rivers includes those with origins in the Flathead Valley and surrounding mountains including the Yaak, Whitefish, Jocko, Little Bitterroot Rivers. B011 Medium Intermontane rivers begin in the Northern Rockies including North, South & Middle Forks of the Flathead River.

Spatial Pattern

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.

  • Classification and Map Identifiers

    1290: Westslope Intermountain Transitional River

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Citation for data on this website:
Westslope Intermountain Transitional River.  Montana Field Guide.  Retrieved on , from