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Montana Field Guide

Montana Field Guides

Small Westside Forested Mountain Streams

Provisional State Rank: S5
(see reason below)

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State Rank Reason
The number of viable stream mile occurrences is unknown, but these stream ecosystems are abundant across the mountain ranges of Western North America, often in protected lands managed by the National Park Service or US Forest Service.

General Description
This ecosystem is found in the mountainous, moderate-to-high elevation (1600-2500m), forested, moderately confined-channel streams of the Middle Rockies Ecoregion, including the isolated mountain ranges. These small-to-medium (2nd to 3rd order, average wetted width =7m, average summer temperature <15°C) moderately flowing streams have permanent flow with strong seasonal variability due to melting snowpack from higher elevation mountainous areas. These streams represent the transition from alpine stream communities to foothills and intermontane rivers, and provide substantial habitat for Montana’s native cutthroat trout populations, which thrive in the cold water temperatures and complex in-stream habitats. The geomorphology of these streams is usually a step-pool configuration with a substrate dominated by boulders, cobbles, and gravel in the short pools. Large woody debris from the surrounding hillslopes can provide significant channel material and additional substrate to these streams.

Diagnostic Characteristics
This moderate gradient, Montane Forested stream type originates in the Northern and Middle Rockies of the upper Missouri and Columbia River Basin. The fish community is the Traditional Trout Stream Assemblage, specifically the Small Trout Stream Assemblage, typified by westslope cutthroat trout and Rocky Mountain sculpin in reference quality Missouri and upper Columbia drainage streams. Unfortunately, the introduced brook and rainbow trout have pushed many native cutthroat trout populations to the brink through aggressive competition (brook trout) and hybridization (rainbow trout). Rocky Mountain sculpin usually persist in all of these types, but single species assemblages of westslope cutthroat trout occur in many streams that have sufficient downstream barriers to prevent the dispersal of the sculpin upstream. Interestingly, these downstream barriers have allowed the persistence of high quality, intact small mountain stream communities by impeding the colonization of introduced species into the pure cutthroat trout areas. A diverse macroinvertebrate community of coldwater stenotherms makes up the Mountain Stream and Medium Mountain Stream Assemblages. The community indicator species are characterized by intolerant, main channel, fast-current mayfly, stonefly and caddisfly species (Baetis bicaudatus, Caudetella spp., Drunella spp., Epeorus spp, Cinygmula, Zapada spp., Megarcys, Doroneuria, Sweltsa, Paraperla, Micrasema, Neothremma, Parapsyche, Neophylax spp., and numerous Rhyacophila species Ggroups) and the cold-water dipterans (Rhabdomastix, Bibiocephela, and Glutops).

In Montana, the Mountain Stream type is reported from over 100 sites within the Northern and Middle Rockies Ecoregions. These include streams in many of Montana’s western mountain ranges, including the Bitterroots, Beaverheads, Flint Creeks, Garnets, Anaconda-Pintlers, Pioneers and the Sapphire Mountains. These ecosystems typically fall within the boundaries of National Forest Service lands and wilderness areas.

Spatial Pattern

Due to the confined valley nature of these streams, the largest management issue involves keeping the riparian zone intact. Disturbances in the riparian zone (e.g., logging) can have severe water quality impacts from bank erosion, sedimentation, increased stream temperatures, silt deposits and loss of large woody debris. Livestock use around the riparian areas can have strong local effects resulting in sedimentation and stream widening at cattle crossings.

  • Classification and Map Identifiers

    1470: Small Westside Forested Mountain Streams

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Citation for data on this website:
Small Westside Forested Mountain Streams.  Montana Field Guide.  Retrieved on , from