A combined catchment and reach-based assessment of historical channel planform change in a UK upland gravel-bed river

Abstract

River channel planform change in upland gravel-bed rivers has consequences for riparian land use, river habitats and flood hazards. Channel planform is predominantly determined by the balance between sediment supply and prevailing discharge regime which is governed by climate and land use, and anthropogenic channel modification. While the nature of channel response to individual factors is in some cases well documented, the complex interaction of theses variables and their relative significance requires further evaluation. This study assesses the nature and extent of historic (1856-1991) channel planform change in the upland catchment of the River Wear, Northern England. A nested research strategy is adopted consisting of catchment wide and reach-scale evaluations of channel planform change using archival maps and aerial photographs. The cause of channel changes is considered with reference to hydrological records (1800-present), catchment land use (metal mining) and accounts of river channel modification (gravel extraction). Comparative historic (reach-scale) and contemporary (trunk channel) data is provided from the upper reaches of the River Tees in Teesdale, Northern England. In the mid nineteenth century a pulse of sedimentation migrated through the upper Wear catchment resulting in the development of eight major sedimentation zones with channel braiding separated by confined single thread reaches. Increased sedimentation was related to a combination of high magnitude floods during the nineteenth century and enhanced sediment supply from extensive metal mining in tributary streams. A decline in flood frequency and magnitude coupled with the cessation of metal mining during the early twentieth century, led to a reduction of sediment supply and lateral channel instability in the catchment. This promoted gravel bar stabilisation through vegetation colonisation, reducing average channel width by 56%. In the sedimentation zones, gravel bar extent declined by up to 87% and this was accompanied by a change to a single thread planform. Gravel extraction operations during the mid-twentieth century lead to local instability and delayed the decline in gravel bar area. In contrast the contemporary channel of the upper Tees is still characterised by localised channel division, around both active gravel bars and vegetated islands. Differences between the two catchments demonstrate the importance of catchment specific controls such as bedrock control and anthropogenic channel interference. A large flood on 30 July 2002 provided significant insight into the mechanism of channel planform change. Substantial increases in gravel bar extents were recorded together with localised changes in flow routing. While local controls such as gravel extraction are significant over decadal time scales, catchment scale controls, primarily changes in the frequency of large floods; determine channel planform over centennial timescales. In order to discriminate ― between local and catchment controls, catchment wide assessments are required.