Anthropogenic sediment traps and network dislocation in a lowland UK river

Farm ponds, reservoirs and in-stream weirs exist in most lowland UK river catchments and often dominate natural features such as lakes, wetlands, floodplains and debris dams. Artificial structures have served multiple purposes, including provision of power for historic flour milling and iron ore crushing and provision of water for medieval fishponds, canals, crop irrigation and potable supply. Although unintentional, they can significantly affect longitudinal connectivity, including sediment delivery pathways, through river catchments.

We report results from three spatially nested case studies that were undertaken in the Rother catchment, ranging in scale from small farm ponds of a few square metres in area, to larger in-stream weirs and reservoirs (locally called ponds). Reservoirs typically trap sediment, decreasing sediment availability downstream, while inducing valley sediment accumulation upstream. We focus on the quantity and particle size characteristics of sediment trapped behind these structures compared to catchment soils and to sediments that are transported through, and deposited in, ‘natural’ gravel-bed reaches.

At all scales our results demonstrate that sediment trapping and release are particle size specific. Fine to coarse sands (125 μm to 2 mm diameter) and coarser sediments are retained behind structures at all scales, while silts and clays (<63 μm diameter) and organic matter are generally depleted in the stored sediment. Even though 75% of the surveyed reservoirs have very low estimated trap efficiencies (<5%), they slowly fill over time with sediment.

An important management question relates to the likely benefits of impoundment, structure or sediment removal, and whether fine (here defined as <63 μm) or coarser (>63 μm) sediment is a priority for management.