The fate of suspended sediment and particulate organic carbon in transit through the channels of a river catchment

Particulate organic matter (POM) transiting through rivers could be lost to overbank storage, stored in-channel, added to by erosion or autochthonous production, or turned over to release greenhouse gases to the atmosphere (either while in the water column or while stored in the channel). In the UK, a net loss of POM across catchments has been recorded, and the aim here was to investigate the balances of processes acting on the POM. This study considered records of suspended sediment and POM flux in comparison to stream flow, velocity, stream power, and residence time for the River Trent (English Midlands, 8,231 km(2)). We show that for the lower two thirds (106 km) of the River Trent, 2% is lost to overbank storage; 10% is lost to the atmosphere in the water column; and 31% is turned over while in temporary storage. Permanent in-channel storage is negligible, and for the lower course of the river, material stored in-channel will have a residence time of the order of hundreds of days between the last flood hydrograph of one winter and the first winter storm of the next winter (usually in the same calendar year). When considered at the scale of the UK, 1% POM in transit would be lost to overbank sedimentation; 5% turned over in the water column, and 14% turned over while in temporary storage. In the upper third of the study river channel, there is insufficient stream power to transport sediment and so in-channel storage or in-channel turnover over to the atmosphere dominate. The in-channel processes of the River Trent do not conform to that expected for river channels as the headwaters are not eroding or transporting sediment. Therefore, the source of sediment must be lower down the channel network.