Sediment loads and sources in heavily modified Irish catchments: A move towards informed management strategies

Abstract

Implementation of the EU Water Framework Directive (2000/60/EC) has forced regulatory authorities to take an integrated view of the link between water quality and catchment management. In Northern Ireland, concern over progressive habitat degradation of one of the Province's prime salmon rivers (the River Bush) provided the stimulus for a sediment monitoring programme. The aims of this study were to quantify instream sediment loads and to identify sources of fine sediment feeding into the river channel. This information would then be used to justify changes in the way the catchment's resources were managed. Sediment loads were assessed at four sampling stations on the River Bush system over a one-year period (between 18/07/02 and 24/07/03) using sediment collection tubes and pit-type traps. These ranged between 0.196 and 4.98 t yr − 1 for bed load and 1.70 and 102 t yr − 1 for suspended load. A combination of desktop studies (erosion potential mapping), monitoring (visual observations, bank erosion magnitude and yield) and analytical work (sediment fingerprinting) was then applied in order to elucidate links between catchment sediment erosion and downstream sediment delivery. Bank erosion was highest in regions of the catchment with the least cohesive bank materials during high flow conditions. Livestock poaching and peak flows caused damage to banks at a localised scale and led to selective patches of bare land being susceptible to further erosion, augmenting the sediment load (approximately 2% of the annual suspended sediment and 60% of the annual bed load). Drainage maintenance work (60% and 30%), forest clearfell (1% and 2%) and ploughed land (37% and 8%) also contributed to the annual suspended sediment and bed load, respectively. Nine key actions were suggested in order to improve habitat quality in relation to sediment supply in the River Bush. These included wetland restoration; prohibiting drainage maintenance work; strategies to control conifer plantation felling; reducing bare ground; livestock access restrictions; construction site management; macrophyte clearance; employment of a river warden and systematic dissemination of project recommendations to the general public to generate community involvement. It was essential that these actions were both cost effective and practical in order to maximise catchment stakeholder uptake.