Two decades of change in beaver-driven hydromorphological impacts and water retention in a temperate lowland forest catchment – Lessons for implementing nature-based solutions in water management

This study assesses long-term changes in beaver-induced hydromorphological impacts and water retention in a small temperate lowland catchment. Field surveys conducted in 2006, 2013, and 2025 in the Krzemianka River basin (northeastern Poland) documented the distribution, geometry, and condition of beaver dams and ponds, including their height, area, volume, and spatial extent of backwater influence. Measurements were obtained through direct field mapping, GPS positioning, and UAV-based analysis for larger floodplain features, providing a nearly 20-year perspective on system dynamics. The results indicate a clear decline in the number of beaver dams (from 22 to 14) and in the length of the river influenced by impoundments (from 2750 m to 1580 m), accompanied by a significant reduction in total pond area and median retention volume. Statistical analyses confirmed significant decreases in pond area and storage capacity over time, while dam height remained relatively stable. Despite this contraction, the system exhibited structural reorganization, with fewer but locally larger ponds in 2025 (maximum volume up to 6300 m³), partially compensating for the reduced number of structures. Many dams were abandoned but continued to influence channel morphology, sediment storage, and local water retention. These findings suggest a shift from extensive to more localized retention patterns, likely controlled by ecological constraints such as resource availability and predation pressure. Importantly, the overall impact on catchment-scale water retention appears relatively stable despite substantial structural changes. This highlights the functional resilience of beaver-engineered systems and underscores their relevance as nature-based solutions. For water management, the results imply that the effectiveness of measures such as beaver dam analogs depends not only on the number of structures but also on their size, configuration, and persistence within dynamically evolving ecological systems.