Sub‐annual spatiotemporal dynamics of sediment and organic matter storage in beaver ponds

High, often nutrient-enriched, sediment loads in highly modified agricultural landscapes cause great ecological damage to receiving surface waters and expense in water resource management.

With their unique dam-building behaviour, the return of beavers (Castoridae) to their former ranges provides channel restoration and eroded sediment trapping potential. However, rates of sedimentation in beaver wetlands vary widely between studies. Further, as rates have generally been determined by averaging multi-year deposit thickness, finer temporal scale sedimentation dynamics and causal relationships are poorly understood.

We build upon a previous proof-of-concept study using sonar to monitor changes in the thickness of sediment and organic matter deposits in beaver ponds, over distances of metres and timescales of months. These spatial and temporal resolutions are finer than those previously published for active beaver ponds.

Of the five study ponds in three separate enclosed beaver cascades in south-west England, three showed net sediment accretion and two ponds showed net erosion during the study. Total Carbon and Total Nitrogen content were higher than similar non-beaver ponds. All ponds showed spatial (≤5 m) and temporal (within 2–4 months) variation in accretion and erosion rates.

The position of ponds in the beaver dam cascades, and the maximum antecedent rainfall accounted for 76% of the variation in average daily elevation change. All ponds showed net accretion between surveys with rainfall intensity <12 mm hr−1. Above this intensity, first ponds in the cascades increasingly showed net erosion, whereas second ponds showed negligible change, and the third pond in a sequence showed increasing accretion.

The results help explain the impacts and dynamics of sediment and associated carbon and nutrient storage in beaver wetlands. Future research can build on these findings over a wider range of non-enclosed, wild beaver cascades to test hypotheses around beaver wetlands' potential to mitigate sediment and nutrient transport, particularly in intensively managed catchments.