Forensic Hydraulic Analysis of Floodplain Connectivity Driven by Historical Beaver Dams in Colorado Headwater Streams

As ecosystem engineers, beavers ( Castor canadensis ) modify river corridor form through dam building. When beavers are removed from a river corridor, their unmaintained dams wash out, altering the stream's hydrologic regime. The assumption that beaver dams increase floodplain connectivity is frequently presumed but has not been directly quantified. To address this gap, we assessed changes in floodplain connectivity caused by the loss of beaver dams at three headwater streams in Colorado, USA. Using a forensic analysis of historical beaver activity, we developed two‐dimensional hydraulic models to compare metrics of floodplain connectivity under historical (beaver‐active) and present (no beaver activity) scenarios. At three simulated flood discharges, we quantified changes in the volume of water on the floodplain, the fraction of flow through the floodplain, the volumetric flux into the floodplain, and mean site and floodplain residence times. The loss of beaver dams decreased floodplain connectivity across all metrics (up to a 96.5% loss in connectivity) except mean floodplain residence time, which increased in the absence of dams. Sensitivity analysis results show that the flow state and condition of beaver dams can have reach‐scale impacts on floodplain connectivity. Notably, while we observed floodplain disconnection due to the loss of beaver dams at each site, the magnitude of change varied depending on both site‐specific characteristics and on the flood magnitude. We conclude that, in headwater streams, beaver dams play an important and quantifiable role in facilitating floodplain connectivity, and floodplain disconnection from the loss of dams has major implications for other ecological and geomorphic floodplain processes. These results are especially timely considering the increasing interest in beaver‐related restoration.