Channel aggradation below Chew Valley Lake, Somerset, U.K.

Abstract

Variations of channel substrate composition are described for a regulated river which has experienced channel change since dam completion. Sediment samples were obtained using bulk (grab) sampling and freeze-coring. Data derived from the two methods are shown to differ. Bulk-sample data describe the bed surface layer; its grain-size variations are related to bedform. Freeze-core data describe the sub-surface structure and the fine size-grades. The gravel framework-supported substrate of the control reaches constrasts with the sand-dominated substrate of the aggraded reach, although in both cases the substrate coarsens upwards. In the former this relates to the development of a sand matrix within the gravel-framework substrate, and in the latter, sand-bed channel, to the progressive addition of a gravel component towards the surface which is related to the process of channel change. At the local-scale, bedform is shown to exert an important control on the character of substrate change induced by flow regulation.