Whole System Ecohydrological Change Following Natural Flood Management and a Five-Year Beaver Reintroduction Trial

Once-common beavers have been absent from the British landscape for centuries, but wild beaver populations have returned in recent years as part of reintroduction schemes, including releases into monitored enclosures. In North Yorkshire, such a release of Eurasian beavers took place in 2019. Using an interdisciplinary combination of traditional (e.g., species surveys, repeat fixed-point photography) and cutting-edge (e.g., mobile LiDAR and eDNA sampling) monitoring and modelling, we report on the effect of those beavers on the hydrology, geomorphology and ecology over a 5-year period, including their interaction with existing natural flood management infrastructure. The beavers constructed six dams after which the hydrological response became less flashy with lower peak flows than in the pre-beaver period. Compared with an upstream gauge, there was a 46.6% reduction in median peak flows, a 417% increase in baseflow (Q95), a 31% median reduction in turbidity and general improvement in other water quality metrics. Beavers did not initially incorporate existing in-stream infrastructure into their dams, which degraded and lost over half of their storage capacity; however, the additional capacity provided by beaver dams mitigated this reduction, and there are recent signs of beavers restoring the functionality and extending the active life of these degraded human-built dams. Aquatic plant diversity increased, and vertebrate species richness was higher than a neighbouring stream; increases were observed from monitoring small mammals, amphibians, dragonflies and damselflies. Bat populations increased at locations and times that exactly corresponded with beaver activity. Our novel interdisciplinary monitoring approach clearly demonstrates the profound and interconnected impacts beavers have upon local hydrology, geomorphology and ecology.