NONSYNCHRONOUS RESPONSE OF ADJACENT FLOODPLAIN SYSTEMS TO HOLOCENE ENVIRONMENTAL-CHANGE

Two adjacent upland floodplains are compared to establish the local re sponse of floodplains to environmental change. Radiocarbon dating, the analysis of sedimentary exposures, terrace mapping, aerial photograph y and archaeological evidence are used to examine Late Quaternary vall ey fill sediments on the Afon Tanat and the Afon Vymwy in the Upper Se vern Basin, Wales, UK. The alluvial stratigraphy of the two floodplain systems consists of Late Devensian (last glacial) fluvio-glacial sedi ments at the valley margins deposited under a braided outwash river re gime. Holocene age terraces, with floodplain and palaeochannel deposit s composed of gravels overlain by silty-sands, are inset into this old er unit and were formed by meandering fluvial channel systems. A combi nation of field and laboratory data demonstrates that from the mid-lat e Holocene the two floodplain systems had divergent development. The A fon Vymwy has remained vertically stable for the last ca. 4000 yrs. wh ereas the adjacent Afon Tanat continued to be vertically and laterally active. Both floodplain systems have been affected by Late Quaternary climatic fluctuations and anthropogenic activity from the Bronze Age to the Roman period, but local geomorphic gradient controls, combined with a possibly greater focus of anthropogenic activity in the Tanat c atchment, may explain the differential evolution of the two systems. T he data demonstrate that multiple reach-scale studies are essential fo r revealing significant stages in the chronology and historical develo pment of fluvial systems.