Creature Feature: Leave it to Beaver: River Restoration partners with Castor canadensis
Beaver (Castor canadensis) are North America’s largest rodent. Once numbering around 400 million, beaver were historically distributed across North America, including northern Mexico. Historic beaver dam density would have been impressive, with estimates of 25 million dams and an average density of 10 dams/km. Beaver and Pacific salmon co-evolved in North American streams choked with dams for at least the last 6 million years.
However, in the 1600s Euro-Americans discovered the incredible quality of beaver fur and a 300-year period of commercial exploitation began. By 1900, beaver populations had been decimated throughout their native range. Fortunately, conservation practices were implemented in response to the near extinction. Beaver now occupy most of their historic range, and populations have rebounded to approximately 10 million in North America.
Beaver are widely considered “Ecosystem Engineers” because they modify the riverine landscape to create their preferred habitat. Their modifications benefit other aquatic and riparian species as well as critical hydrological and geomorphic processes. Beaver build dams across small and medium-sized streams and on side channels in larger streams to create ponds. Ponds provide protection from predators, expand the foraging area, allow for food storage over-winter, and provide a safe location to build lodges.
There are many positive benefits of beaver pond complexes for fish species. The impounded water provides critical rearing habitat for juvenile salmonids, particularly endangered coho. A recent study in the Stillaguamish Watershed in western Washington found an 86% reduction in coho smolt production potential from historic levels. The majority of the loss was attributed to beaver dam removal. Beaver ponds also greatly enhance channel complexity with spawning gravel deposition upstream of ponds and plunge pools scoured downstream of dams. Sediment and water retention from dams enhance existing floodplain connection and create new floodplain surfaces which provides high-flow refugia for salmonids.
Dams are sponges—absorbing sediment and water which elevates the water table. This in turn expands the riparian corridor, elevates base flow, and provides critical refugia from extreme water temperatures. Ponds and associated riparian expansion also benefit mammals, reptiles, amphibians and birds. Possibly one of the most important benefits of beaver dams is their role in combating the effects of climate change. The cumulative increase in water retention in a watershed with high dam densities attenuates peak flows, providing higher base flows during the dry summer months.
For these reasons, river restoration practitioners have begun partnering with beaver to rehabilitate degraded rivers. This style of restoration was developed in Bridge Creek, Oregon, for example, by fisheries scientists and geomorphologists from the National Oceanographic and Atmospheric Administration (NOAA), Eco Logical Research Inc., and Utah State University. Bridge Creek was deeply entrenched and disconnected from its historic floodplain due to poor land management. Vertical downcutting left the stream with a simplified channel and poor steelhead habitat. Beaver were already present in Bridge Creek but their dams didn’t persist; the narrow trench didn’t allow for flow dispersal across the floodplain and the poor riparian area didn’t provide robust dam building material. This left too much stream power in the channel and dams failed during high flows.
In response, the researchers developed methods of re-enforcing existing dams and installing Beaver Dam Analogs (BDAs—a series of fence posts pounded into the riverbed, interwoven with willow branches) to rehabilitate Bridge Creek. This creates a semi-permeable dam surface which is then reinforced with rocks and mud to mimic beaver dams. In many cases beaver treated the BDAs on Bridge Creek like their own, providing dam maintenance and expansion. Over the last five to six years this method has had tremendous success, elevating the height of the stream bed, creating and enhancing floodplain connection, expanding the riparian corridor and creating critical habitat for steelhead. This has translated into a significant increase in steelhead production. Additionally, through an extensive series of fish tag readers, they have established that dams and BDAs do not impede the movement of juvenile and adult salmonids.
Beaver are a critical part of the riverine ecosystem. They can help us to restore and rehabilitate rivers and may help combat the effects of climate change. There are increasingly more resources on the ecological benefits of beaver and how to implement beaver-assisted restoration. To learn more, visit www.beaver.joewheaton.org.
Elijah Portugal is a Fluvial Geomorphologist with the Fluvial Habitat Center, Utah State University.