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].