In a nutshell

• Near extirpation of beavers from the Pacific Northwest has contributed to a loss of habitat complexity in many salmon-bearing rivers
• Rebounding beaver populations are re-engineering some rivers, allowing recovery of some of the lost habitat features important for stream-rearing fish
• Highly degraded rivers with incised channels are difficult for beavers to dam, thereby impeding the restoration of their habitat-forming activities
• Restricting harvest of beavers and installation of beaver dam analogs greatly enhances the restoration of beaver-engineered fish habitat, even in highly degraded sites
• Growth, survival, and production of steelhead trout were all substantially enhanced within a few years as a result of increased beaver activity, suggesting that recovery of beaver populations may in turn benefit the recovery of ESA-listed steelhead populations

Recent work carried out in Oregon’s Bridge Creek, a degraded 28-mile tributary of the John Day River and part of the greater Columbia River drainage basin, highlights the pronounced effects that ecosystem re-engineering by beavers have on riparian landscape dynamics and fish. Bridge Creek is part of NOAA’s Intensively Monitored Watershed program, in which selected streams and the fish populations they support are closely monitored following restoration to determine recovery success. In 2009, the Oregon Department of Fish and Wildlife and the Bureau of Land Management imposed restrictions on beaver trapping and livestock grazing, respectively, within the boundaries of the watershed to promote the growth of the regional beaver population. Nicolaas Bouwes and several colleagues used this opportunity to examine the impacts that beaver activity have on steelhead trout populations in Bridge Creek by mimicking and facilitating dam construction through the installation of over 120 “beaver dam analogs” (BDAs). These BDAs – essentially lines of wooden fence posts interwoven with willow branches, with rocks and mud piled on the upstream side – were designed to partially simulate natural beaver dam complexes, but also to serve as stable foundations on which beavers could anchor dams, as scouring flows in deeply incised rivers can impede their ability to construct permanent dams.

Before 2009, the main channel along much of the course of Bridge Creek was shallow, narrow, and supported little bank vegetation, as the lower water levels reduced tree- and shrub-root accessibility to groundwater; lower water levels, along with an absence of shade from overhanging vegetation, led to warmer water temperatures. Moreover, the combination of low water levels and absence of natural flow obstructions – ie beaver dams – inhibited the formation of deeper pools. “Channel incision is a global problem and has resulted in simplified streams, a loss of floodplain vegetation, and poor fish habitat,” explains Bouwes, “and natural recovery of an incised channel to its floodplain can take centuries. Restoration approaches to restore incised streams is extremely expensive and highly uncertain, so we wanted to test the idea that beavers could be an inexpensive yet effective means to help recover incised streams.” By 2013, 171 beaver dams had been constructed in the watershed, a roughly eight-fold increase over the average number of dams present prior to the installation of the BDAs.

Introduction of the BDAs and the subsequent natural dams built by beavers dramatically altered both the physical and hydrological characteristics of the treated reaches: dammed stretches were wider and deeper than un-dammed reaches within just a few years following BDA installation, and featured higher densities of streambank vegetation and cooler water temperatures. Higher water levels also increased the frequency of floodplain inundation and enhanced overall landscape complexity through the creation of numerous side channels and backwaters. “We were surprised at how quickly sediment collected behind the beaver dams,” says Bouwes. “This, along with increased surface water elevations from the dams, allowed water to dissipate onto floodplains during high water, which created more channels and wetted surfaces that allowed for more vegetation like willow to grow. The increased water elevations also increased the height of the water table. We also found cold upwellings below beaver dams, which had a cooling effect in reaches with beaver dams.”

The transformation of hydrologic dynamics generated by the increasing presence of beaver dams, both real and simulated via the BDAs, had rapid and marked effects on steelhead trout populations; although growth rates slowed somewhat, fish densities were higher in pool environments, juvenile survival rates increased by 52% compared to controls lacking BDAs, and production of juvenile steelheads rose by approximately 175% over the 4 years following installation of the BDA structures, resulting in overall higher net production of steelhead trout. Moreover, beaver dams did not appear to impede the upstream movement of adults, nor the downstream movement of juveniles. According to Bouwes, “fish need habitat to forage, avoid predators, and to rest; more complex habitat allows fish to do these activities with less time spent migrating between habitat patches than in simpler habitat. The ponds and the raised water table act as reservoirs that collect water during spring runoff and release it more slowly throughout the summer. Nutrients and organic material – leaves and branches, for instance – are processed differently in ponds than in free flowing streams, which in turn affects the stream food web, such as the aquatic insect community. The ponds, side channels, and canals not only provide more fish habitat but can make the stream more interesting from a fish’s perspective, in that there are more types of habitat than in a stream that is completely free-flowing.”

The results demonstrated in this and similar studies provide strong evidence that beaver extirpation has had detrimental impacts on the productive capacity of many salmon- and trout-bearing rivers. In addition to ecosystem benefits, “restoration by beavers” represents a substantially more cost-effective approach to improving riparian habitats generally, and fish habitat quality specifically, than traditional restoration techniques. “We spend over a billion dollars every year in this country on stream restoration, but we do not have much evidence – mainly because we spend very little on monitoring – as to how effective those efforts are on their target populations, such as fish. Implementing large-scale restoration experiments like this provide more information about funding efficiency when trying to restore and rehabilitate the huge number of miles of degraded streams,” points out Bouwes. “If we can help beavers get established, for instance, they can do a lot of the heavy lifting for us to restore streams, saving money that can be used for other conservation efforts.”

Science Spotlight by Ken Ferguson

More salmon science in the news:

• Phys.org: Dam good! Beavers may restore imperiled streams, fish populations