Spatio-temporal sedimentation patterns in beaver ponds along the Chevral river, Ardennes, Belgium

With the recovery of the European beaver (Castor fiber) and their capacity to engineer fluvial landscapes, questions arise as to how they influence sediment transport, including the spatio-temporal trends and patterns of sedimentation in beaver ponds. The Chevral river (Ardennes, Belgium) contains two beaver dam sequences, which appeared in 2004. Volumes of sediment deposited behind the dams were measured, and grain-size distribution patterns were determined. Flow discharges and sediment fluxes were measured at the inflow and outflow of each dam sequence. Between 2004 and 2011, 1710.1m3 of sediment was deposited behind the beaver dams, with an average sediment thickness of 25.1 cm. The thickness of the sediment layer was significantly (p<0.001) related to the area of the beaver ponds. Along the stream, beaver pond sediment thickness displayed a sinusoidal deposition pattern, in which ponds with thick sediment layers were preceded by a series of ponds with thinner sediment layers. A downstream textural coarsening in the dam sequences was also observed, probably because of dam failures subsequent to surges. Differences in sediment flux between the inflow and outflow at the beaver pond sequence were related to the river hydrograph, with deposition taking place during the rising limbs and slight erosion during the falling limbs. The 7-year-old sequences have filtered 190.19 ton of sediment out of the Chevral river, which is of the same order of magnitude as the 374.4 ton measured in pond deposits, with the difference between the values corresponding to beaver excavations (60.24 ton), inflow from small tributaries, and runoff from the valley flanks. Hydrogeomorphic effects of C. fiber and Castor canadensis activity are similar in magnitude. The detailed analysis of sedimentation in beaver pond sequences confirms the potential of beavers to contribute to river and wetland restoration, and catchment management.