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Montana Field Guides

Large Valley Intermountain River

Provisional State Rank: S4
(see reason below)

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State Rank Reason
Diversion, flood control and hydro dams have significantly altered large portions of this Large River Ecosystem in MT.

General Description
This aquatic ecosystem is found widely throughout the mainstem upper Columbia River basin of the Intermountian rockies and Pacific northwest. Abell et al. (2000) delineated two freshwater ecoregions (FWE) for western Montana, the glaciated (FWE 120) and non-glaciated Columbia River basin (FWE 121). The later FWE delineates the unglaciated portion, or lower two-thirds of the Columbia River basin (McPhail and Lindsey 1986). These large (7th stream order and larger), cold-water rivers have low to moderate gradient with elevations below 1000m and are 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, sand and gravel in runs and pools, with gravel and/or finer-textured side channels.

Diagnostic Characteristics
These ecological systems are 7th stream order and larger (>200 river miles) cold-water rivers that have low to moderate gradient and elevations below 1000m. Diagnostic fish species include members of the upper Columbia, Large Valley Intermontane River Community. This assemblage is dominated by large bodied sucker, trout and minnow species, in order of occurrence frequencies : Largescale Sucker, Longnose Dace, Mountain Whitefish, Longnose Sucker, Redside Shiner, Northern Pikeminnow Bull Trout, Slimy Sculpin, Westslope Cutthroat Trout, Burbot, Rocky Mountain Sculpin and the Peamouth. Macroinvertebrate Species consist 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 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 the following indicator species: Acentrella, Acerpenna, Hydropsyche, Optioservus, Baetis tricaudatus, Brachycentrus occidentalis, Helicopsyche borealis, Corynoneura, Prosimulium, Amiocentrus aspilis, Lara, Phaenopsectra, Plauditus. 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 Large Intermountain Valley River type occurs in the Clark Fork River from the Alberton Gorge Area to the confluence with and including the lower Flathed River.

Spatial Pattern

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 such as below Libby Dam to the Idaho border 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.

  • Classification and Map Identifiers

    1150: Large Valley Intermountain River

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