An assessment of the impact of upland afforestation on lowland river reaches: the Afon Trannon, mid-Wales

Abstract

Upland afforestation in the UK was the largest rural land-use change last century. As a consequence, the bed load yields of upland catchments increased substantially. Circumstantial evidence suggests that, when this increased load reaches the lowland catchment, it is deposited, triggering flow diversion and channel instability. However, despite the widespread adoption of this theory, it remains to be properly tested. This paper uses aerial photograph analysis and contemporary channel digital terrain models (DTMs) to assess this concept on the Afon Trannon, mid-Wales, the upper catchment of which was afforested between 1948 and 1978. Construction of a sediment budget demonstrates that upland catchment bed load yields are only equivalent to localized inputs of gravel from bank erosion and are therefore unlikely to be totally responsible for producing the high rates of channel change observed in some lowland reaches. Channel instability appears to be more related to the nature of the local bank input as those reaches where gravel is not present in the bank material are stable. Additionally, flood magnitude and frequency are shown to have increased since 1988, a factor of at least equal importance to that of bed load yields from either upland catchments or local bank erosion. It is concluded that, in contrast to previous work, afforestation in the uplands has only a minor influence on downstream reaches of the Afon Trannon. Situations where upland afforestation may have a greater impact on downstream reaches are discussed and the implications for best management practice explored.