Map from the AWL GIS lab.

Pole Creek and Long Creek start in adjoining drainages in southwest Montana’s Gravelly Mountains. The headwaters of the two streams are nearly identical. Both drainages begin in high subalpine basins and flow through open sagebrush foothills before joining to form one of the largest tributaries to the Red Rock River. That’s where the similarities end. Pole Creek flows from one deep pool to another, interspersed with clean gravel riffles. Its cool waters are shaded by thick streamside stands of willow. In contrast, Long Creek’s channel is wide and shallow. The water flowing over its silt choked streambed quickly heats up in the August sun. During the winter, water in Pole Creek’s pools remains liquid under a sheet of ice, while water in Long Creek is frozen solid.

The difference in the two streams’ physical character is reflected in the vibrancy of the biological systems they each support. Pole Creek’s deep pools sustain a healthy population of trout and the aquatic insects that feed them. Riparian vegetation fills the floodplain. In contrast, Long Creek’s scattered trout are stunted from a lack of food. More arid plants, such as sagebrush and bunchgrass, have replaced the lusher willow and sedges normally found along most mountain streams. The final difference in the two streams’ biological community also explains their general disparity: Pole Creek has an active beaver colony, while Long Creek does not.

Beaver, and the dams they build, have a positive impact on how a stream functions. The ponds created behind beaver dams slow down stream velocity, reducing erosion and trapping sediment that would otherwise be flushed down stream to clog important fish spawning gravel. Beaver ponds also raise the water table, increasing the amount of water available for riparian vegetation and helping to maintain stream flows during the dry summer months. Over time, the ponds fill with trapped sediment, creating lush stream-side meadows that support willows, aspen, and other water loving vegetation. The roots of these trees anchor the sediment, further stabilizing the stream bed, while the trees’ shade helps to keep water temperatures cold for native trout and aquatic insects.

When beaver are removed from a stream, stream velocity increases, therefore eroding banks and straightening the stream channel. The eroding channel gradually cuts below the flood plain dropping the overall water table. As the water level drops, the soil in the flood plain dries out and the riparian vegetation dies. With no beaver ponds to retain water, spring run-offs rush downstream unimpeded and the groundwater in the floodplain is no longer sufficient to maintain late summer stream flows. Without beaver, a stream that was once made up of productive, cool-water pools can become warm, shallow, and silty.

In order to restore native fish habitat and recently re-introduced arctic grayling in the Upper Ruby River and its tributaries, American Wildlands is working with the Forest Service to restore depleted beaver populations. While some of the Ruby’s tributaries already support beaver, many tributaries have lost them to historic over-trapping. Not surprisingly, those tributary streams with active beaver colonies also support the most vibrant grayling populations.

Beaver restoration is surprisingly simple—feed them and they will come. This summer American Wildlands and volunteers, working with Forest Service ecologists, will haul aspen trees to unoccupied stream reaches near active beaver colonies to encourage beaver to re-colonize new areas of the Ruby. Not only is aspen a favored food for beaver, it supplies the large pieces of wood necessary to construct dams that are strong enough to withstand spring runoff. In addition to encouraging beaver to recolonize parts of the Ruby, we may also transplant beaver from agricultural areas where their dams are considered a nuisance.

As in most cases, nature really does know best. By restoring beaver populations, American Wildlands is helping to ensure cleaner, colder, and more productive fish habitat in the Northern Rockies.