Production, storage and output of coarse upland sediments: natural and artificial influences as revealed by research catchment studies

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Abstract
Rates of sediment production in UK river basins are generally low; however, upland source areas control both runoff and sediment volumes to he routed through larger basins and there are indications that development pressures on land in upland areas have far-reaching implications. Output rates for bedload from semi-natural grassland and commercially-afforested catchments are now available for a ten-year period at the Plynlimon catchments, mid-Wales. The influence of forestry drainage practice is to increase bedload yields sixfold; suspended load yields are doubled. In an attempt to predict yields and to investigate the operation of storage and transport processes on bedload a regression analysis was conducted, using the weight of bedload trapped as a dependent variable, and selected climatic supply and transport factors as independent variables. The climatic supply factors chosen are shown to have little influence on yield, relative to transport factors, despite evidence of supply-limited yields. Tracer studies indicate that channel storage of material exercises a critical control on yield, a factor which is not accommodated by the climate-based supply indices. Implications for field measurement techniques are discussed.