Partnering with beavers to restore ecosystems

Beaver dams and overbank floods influence groundwater–surface water interactions of a Rocky Mountain riparian area, Westbrook et al. 2006

This study provides empirical evidence that beavers influence hydrologic processes in riparian areas. Conducted at the headwaters of the Colorado River in the Rocky Mountains, the study examines patterns from two beaver dams of surface inundation, groundwater flow, and groundwater level dynamics. The authors observe that :

Beaver dams on the Colorado River caused river water to move around them as surface runoff and groundwater seepage during both high- and low-flow periods. The beaver dams attenuated the expected water table decline in the drier summer months for 9 and 12 ha of the 58 ha study area [Westbrook 2006: 1] … by providing a constant supply of water to the riparian area via surface and subsurface flow paths [Westbrook 2006: 10].

In both cases [both dams], water left the Colorado River, flowed across the floodplain and terrace, and then back to the river far downstream of the dams [Westbrook 2006: 11].

Noting that the current beaver population is but a small fraction of what it was before Europeans settled the west, the authors state that:

If the results of our intensive study were extrapolated to a time of more abundant beaver then the magnitude of their hydrologic effects may have encompassed nearly the entire study area. It is easy to visualize abundant beaver as key drivers of hydrologic processes in mountain valleys and other unconfined stream valleys throughout North America [Westbrook 2006: 10].

The significance of this study is that beaver dams can maintain the water table in forests, creating resilience to drought. Beaver dams do this by causing water to overflow the banks of the river and spread over a greater surface area. More effective even than any given rain event, “overbank flood events have generally been regarded as the main hydrologic mechanism for replenishing groundwater and soil water in riparian areas” [Westbrook 2006: 8].

Modeling intrinsic potential for beaver (Castor canadensis) habitat to inform restoration and climate change adaptation, Dittbrenner et al. 2018

Beavers are recognized for their ability to restore floodplain hydrology and biological function, yet finding suitable places for their reintroduction remains a conservation challenge. The goal of this study was to identify places in the Snohomish River basin of Washington state suitable for beaver reintroduction.

Because of their abilities to modify streams and floodplains, beavers have the potential to play a critical role in shaping how riparian and stream ecosystems respond to climate change. The Pacific Northwest of the United States is experiencing increases in annual air temperature and decreases in snow pack and summer precipitation, resulting in lower base flows, particularly in streams that rely on late season snowmelt. Climate shifts have altered stream-temperature regimes to the detriment of cold-water fishes, including Pacific salmon. Recent increases in winter precipitation and storm magnitude have increased the potential for stream scour, channel incision, and floodplain disconnection, thereby promoting the drying of adjacent riparian areas [Dittbrenner 2018: 2].

By damming streams, beavers create pond and wetland complexes that increase spatial heterogeneity and geomorphic complexity, species and habitat diversity, and therefore ecosystem resilience to climate-induced environmental change. Beaver impoundments slow stream velocity allowing sediment suspended in the water column to settle, aggrading incised stream systems, and reconnecting streams with their floodplains. The increase in surface water promotes groundwater recharge, storage, and supplementation during base flows. The increased geomorphic complexity also promotes higher thermal variability and coldwater refugia in deeper waters and in areas of downstream upwelling [Dittbrenner 2018: 2].

To qualify as a suitable site for beaver reintroduction, a site needs to be intrinsically suitable beaver habitat and clear of competing human interests.

Of 5,019 stream km assessed in this study, just 33% had moderate or high intrinsic potential for beaver habitat. “Of the riparian areas around streams with high intrinsic potential for beaver, 38% are on public lands and 17% are on large tracts of privately-owned timber land” [Dittbrenner 2018: 1], while the rest was on human-dominated landscapes (agricultural, industrial, residential, etc.). Thus, the areas available for beaver reintroduction are limited. Even so, the authors argue that beavers can play a critical role in adapting to climate change, and they propose that watersheds dominated by public ownership, “provide ample opportunities to test how beavers can be reintroduced into landscapes where they are absent or at low population levels” [Dittbrenner 2018: 11].

Beaver restoration would reduce wildfires, Maughan 2013

Politicians often call for logging and fuel reduction to prevent future wildfires. However, it’s not good logging trees that are burning in such fires so much as cheatgrass, annual weed, dry brush and dead weeds. Reintroducing beaver to create ponds could raise the water table, increase humidity in the drainage area (thus reducing burn intensity) and provide a refuge for animals during a fire.