Mapping valley bottom inundation patterns from beaver dam activity: A potential proxy for hydrologic inefficiency

For centuries, streams and rivers have been altered and degraded such that the conveyance of water downstream is unnaturally efficient, often to the detriment of other biophysical processes that maintain healthy riverscapes. Structural elements, such as beaver dams, can impact hydraulics and alter downstream water conveyance. While the hydraulic, hydrologic, geomorphic, and ecological effects of beaver dams have been quantified at individual study sites, study methods are often cost-prohibitive and complicated, making them less practical for monitoring at large spatial scales and in diverse settings. We mapped inundation extent and type (free flowing, ponded, and overflowing) in beaver dam complexes in diverse hydrogeomorphic settings as a simple method to monitor beaver-influenced riverscapes based on delineating inundation patterns. Our mapping of over 75 inundation events at 37 sites suggests that, on average, under undammed conditions valley bottom inundation ranged from 2.7% - 17.4% (mean 6.8%) whereas under dammed conditions the same sites had valley bottom inundation that ranged from 9.5% - 47.5% (mean 23.2%). We observed that, when beaver dams are present, roughly half of surface water inundation is converted from a free-flowing type to ponded and overflow types. This research also reveals that the focus of most previous beaver dam studies on low gradients and low stream orders is unnecessarily restrictive. We report similar magnitudes of influence in steeper gradient riverscapes as well as in beaver-modified floodplains and anabranches of higher-order rivers that are typically considered to be too large for beaver dams. While the quantification of inundated area and type presented here is valuable as a simple metric, we postulate that delineation of inundation type and extent can be used as a practical proxy for physical processes and indicators of riverscape health such as longer and more varied water residence times (i.e., hydrologic inefficiency).