Deposition and storage of fine-grained sediment within the main channel system of the River Tweed, Scotland

This paper assesses the importance of deposition and storage of fine-grained (c. < 150 µm) sediment on the floodplains and beds of the main (non-tidal) channels of the River Tweed (4390 km2), Scotland, and two of its tributaries (River Teviot and Ettrick Water). Caesium-137 analysis of floodplain sediment cores has been used to estimate average rates of overbank sedimentation during the last 30 to 40 years. Average values for individual transects ranged from 0·16 to 2·18 kg m−2 a−1 (0·13 to 2·2 mm a−1). The mean for the 10 transects investigated was 1·29 kg m−2 a−1 (1·3 mm a−1). The total amount of fine sediment deposited was estimated to be about 44 000 t a−1. The fine-grained sediment stored in the channel bed was quantified using resuspension techniques. Average values for individual sites ranged from 0·12 to 0·96 kg m−2. The mean for the 10 sites investigated was 0·56 kg m−2. The total amount of sediment stored on the channel bed of the main channel system at the time of sampling was estimated to be about 4300 t. Comparison of these estimates of floodplain and channel storage with the estimated suspended sediment load for the River Tweed at the downstream gauging site at Norham, indicates that floodplain sedimentation and channel bed storage represent about 40 and 4 per cent, respectively, of the annual load of fine sediment delivered to the main channel system. Erosion of channel banks will reintroduce the equivalent of about 30 per cent of the floodplain-deposited sediment back into the channel. The residence time of the fine-grained sediment stored on the channel bed is probably less than one year, but that of sediment deposited on the floodplain is likely to be considerably longer. Conveyance losses associated with overbank deposition have important implications for the routing of sediment through fluvial systems and the interpretation of downstream sediment yields.