Rates of contemporary overbank sedimentation and sediment storage on the ¯oodplains of the main channel systems of the Yorkshire Ouse and River Tweed, UK, have been investigated, so that the estimates of sediment ¯ux obtained for these rivers within the framework of the LOIS Community Research Programme can be considered in relation to their overall suspended sediment budgets. Caesium-137 measurements have been used to establish the recent rates of accumulation of ®ne-grained overbank sediment deposits associated with sediment cores collected from a series of transects across the ¯oodplains of the study rivers. Average annual sedimentation rates over the past 30±40 years estimated for individual cores ranged from 0 to cX 1Á60 g cm À2 yr À1 in the Ouse Basin and from 0 to cX 0Á70 g cm À2 yr À1 in the Tweed Basin. In general, highest accumulation rates occur closest to the channel and values decrease with increasing distance from the channel, although for some transects very low accumulation rates also occur near the channel. Average overbank sedimentation rates for individual transects ranged from 0 . 010 to 0Á554 g cm À2 yr À1 in the Ouse Basin and from 0 . 016 to 0Á218 g cm À2 yr À1 in the Tweed Basin, with the mean for all transects being 0 . 206 and 0Á129 g cm À2 yr À1 , respectively. The total annual storage of ®ne-grained sediment on the ¯oodplains bordering the main channel systems of the study rivers have been estimated by extrapolating the sedimentation rates derived for the individual transects to the adjacent ¯oodplain reaches and calculating the total mass of sediment deposited upstream of the tidal limits. The resulting values are 70 970 t for the River Ouse (including the River Wharfe) system and 43 920 t for the River Tweed. These values of ¯oodplain storage have been compared with the annual suspended sediment loads at downstream gauging sites for 1995 and 1996 provided by the LOIS Core Monitoring Programme for the River Ouse and estimated from Harmonized Monitoring Programme data for the River Tweed. This comparison indicates that 39 and 40% of the total suspended sediment delivered to the main channel systems of the rivers Ouse (to Skelton) and Tweed (to Norham), respectively, is deposited and stored on their ¯oodplains. For the River Wharfe, the conveyance loss is c. 50%, and this higher value is thought to re¯ect the low sediment loads recorded for 1995 and 1996, which are likely to underestimate the longer term average values with which the estimates of sediment storage should be compared. The results obtained have important implications for the interpretation of downstream suspended sediment ¯uxes in the study rivers in terms of sediment mobilization and transfer within the upstream drainage basin.