Patterns of channel adjustment to impoundment of the upper River Spey, Scotland (1942-2000)

Over the last 30 years, understanding of how river channel morphology adjusts to upstream impoundment has been improved considerably with clearwater erosion close to the dam and aggradation downstream of-unregulated tributary confluences being identified. In a UK context our understanding of channel response to impoundment has been based principally on analysis of a number of regulated rivers using a time-space substitution approach but the temporal pattern of channel change was not fully observed. This paper details channel change over almost six decades following impoundment of the River Spey, Scotland, primarily via the use of cartographic and aerial photographic evidence. Channel change initially involved the formation of tributary confluence bars followed by bench development and vegetation colonization and development on the benches and gravel bars. Sixty years after impoundment, the channel is still adjusting its form at distance from the dam to be in quasi-equilibrium with its regulated flow regime; indeed adjustment was most rapid between 1989 to 1995. This accelerated adjustment is due to the occurrence of a greater number of large and geomorphologically significant flood events in this period. Sediment supply limitation from unregulated tributaries will have stemmed the rate of narrowing and bench development prior to 1989. This spatial and temporal pattern of adjustment supports, for the most part, the model advocated by Pelts (Progress in Physical Geography, 1979, Vol. 3, pp. 329-362). The wider implications of the channel change for river conservation and management are addressed. Copyright (C) 2003 John Wiley Sons, Ltd.