Historical channel change and sediment dynamics in a heavily managed and protected small catchment: The Afon Dysynni, north Wales

European rivers have, in general, incised and narrowed over the twentieth century as a result of a wide range of climatic and anthropogenic factors (flood frequency and magnitude changes, regulation, gravel extraction, land use change, historical metal mining). In Welsh rivers these factors have led to decreases in exposed riverine sediment (ERS) area during that time. Such morphological changes and sediment dynamics can significantly affect flood risk through changing channel capacity. The Afon Dysynni catchment (131 km2), North Wales, is particularly prone to both fluvial and coastal flooding. During the 18th and 19th centuries, the river was heavily managed through extensive flood embankments and land drainage works, but is currently protected through
National Park, Landscape of Historic Interest, UNESCO Dyfi Biosphere and Sites of Special Scientific Interest designations. Geomorphological mapping, air photo interpretation and historical and contemporary OS map analysis (1888, 1891, 1901, 1964, 1974, 1981 and 2012) was undertaken in order to assess lateral channel migration rates, ERS changes, sinuosity, channel width and sediment dynamics. Anthropogenic activities during the last 300 years have resulted in the progressive confinement of the river along a significant proportion of its length, and have limited the floodplain width, across which the river is able to freely meander, to ~ 70 m. In its middle, unconfined reach (~ 2 km in length), the river has been laterally active, displaying rates of bank erosion of > 3 m yr-1(1964-1974). Between 1888 and 2012 ERS area decreased from 43,506 m2 to 8425 m2. Palaeochannels, located outside the flood embankments, indicate the river was highly laterally active prior to confinement. This study demonstrates that the response of the Afon Dysynni is typical of other Welsh rivers in protected areas, where the legacy of historical anthropogenic activity continues to exert its influence on the rate of natural process change.