The channel-bed materials have a marked influence on the biota of lotic systems. Experimental studies suggest that flow regulation, specificially the reduction in magnitude of flood events, may lead to the infiltration of fine particles into, and the concentration of these sediments within, open framework gravels downstream from tributary sediment sources. This paper examines the distribution of fines (sub-2 mm), within channel substrate gravels along two regulated rivers in upland Britain.
Bulk- and freeze-sample techniques were used: the latter yielded volumes of fines five times those of the former. This emphasizes the problem of wash-out during bulk-sample collection, especially in compact, coarse gravel substrates. Nevertheless, the same spatial pattern was revealed by both data sets. Along both regulated rivers, the proportion of the substrate finer than 2 mm exceeds 20 per cent, by weight, below tributary confluences, contrasting with values for non-regulated sites of less than 5 per cent. Both the amount and size of matrix sediments decrease downstream from the confluences. The spatial scale of the impact is small and the resultant ecological consequences of substrate sedimentation are likely to be localized.