Dynamics of fluvial fine sediment transfer in the River Esk, North Yorkshire, UK

Abstract

A full understanding of the complexities of fluvial suspended sediment dynamics requires a holistic approach. The study presented here addresses fine sediment transfer in the River Esk catchment, which is suffering from the adverse effects of increased fine sediment flux, in particular the silting up of salmon and sea trout spawning habitats and pearl mussel beds. Time-integrated mass flux sampling was used to monitor relative suspended sediment loads at 17 locations in the catchment. This was combined with water sampling, turbidity and stage monitoring at two locations which provided measurements of suspended sediment and its relation to flow at a high temporal resolution. A GIS database for the main Esk and major tributaries was constructed, containing channel and riparian attributes relevant to fine sediment transfer. These were obtained from detailed attribute mapping. The monitoring results were related to these mapped characteristics Relative specific suspended sediment yields were found not to be solely related to catchment area, but were more dependent on the varying substrates and the land use intensity in the sub-catchments. Suspended sediment transport was supply-limited and subject to exhaustion effects. The most common type of hysteresis was clockwise, which occurred in large events where sediment supplies were abundant, while anticlockwise hysteresis was restricted to events where peak suspended sediment concentration was low. It was inferred that the dominant source of fine sediment to the Esk is from in-channel sources, primarily bank erosion and storage of sediment on the channel bed. Livestock poaching increased bank erosion in one reach of the main Esk and in several tributaries. Sediment transport in the Esk is sensitive to high flows, when sediment from in-channel sources is flushed through the system. Sudden increases in suspended sediment concentrations between and within storms showed recharge of sediment supplies to be episodic, probably related to bank failure or removal of debris jams. Management of fine sediment in the Esk should be targeted at reducing in- channel sediment supply to the flow in the upper main channel and in the tributaries of Glaisdale, Great Fryup and Butter Becks.