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 graine d (c. < 150 mu m) sediment on the floodplains and beds of the main (non-tid al) channels of the River Tweed (4390 km(2)), Scotland, and two of its trib utaries (River Teviot and Ettrick Water). Caesium-137 analysis of floodplai n sediment cores has been used to estimate average rates of overbank sedime ntation during the last 30 to 40 years. Average values for individual trans ects ranged from 0.16 to 2.18 kg m(-2) a(-1) (0.13 to 2.2 mm a(-1)). The me an 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 quanti fied using resuspension techniques. Average values for individual sites ran ged 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 th e main channel system at the time of sampling was estimated to be about 430 0 t. Comparison of these estimates of floodplain and channel storage with t he 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 cha nnel banks will reintroduce the equivalent of about 30 per cent of the floo dplain-deposited sediment back into the channel. The residence time of the fine-grained sediment stored on the channel bed is probably less than one y ear, but that of sediment deposited on the floodplain is likely to be consi derably 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. Copyright (C) 1999 Jo hn Wiley & Sons, Ltd.