Title: Provenance of Interstitial Sediment Retrieved from Salmonid Spawning Gravels in England and Wales
Author: D E Walling
Author: A L Collins
Author: GK McMellin
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_1643, Representation ID: 609, Object ID: 2750
The siltation of spawning gravels has been increasingly cited as an important cause of the declining success of salmonid fisheries reported for many years in England and Wales. Existing remedial measures, including gravel cleaning and the use of egg boxes for restocking, typically yield short-term benefits and are constrained by cost and practical difficulties. It is therefore generally recognised that programmes aimed at preventing or reducing siltation afford a more appropriate and sustainable management strategy for addressing the problem. The implementation of effective strategies for the prevention or reduction of spawning gravel siltation is heavily dependent upon an understanding of the source of the increased fine sediment loadings in those river basins where the siltation of salmonid spawning gravels represents an environmental problem. Without reliable information on sediment provenance, the resources available for preventing or reducing the siltation of spawning gravels cannot be properly targeted. Assembling accurate information on catchment sediment sources represents a difficult task, because of the limitations and uncertainties associated with traditional measurement and monitoring procedures. Due to these constraints, the fingerprinting approach is increasingly recognised as an alternative indirect method for establishing the source of the sediment transported by a river. Given the need to develop sediment control strategies for salmonid spawning gravels, this report presents the findings of a reconnaissance investigation, based on the use of the fingerprinting approach, aimed at assembling information on the relative importance of surface and channel/subsurface sources in accounting for the provenance of interstitial fine sediment samples retrieved from the salmonid spawning gravels in 18 rivers in England and Wales. The findings verify the utility of the fingerprinting approach for investigating the source of interstitial fine sediment accumulating in salmonid spawning gravels. The primary source of the interstitial sediment collected from the study rivers was found to vary regionally in response to a number of controls. For example, channel banks are important sediment sources in the south-west of England and south Wales, where river channels are heavily incised and the trampling and degradation of channel margins of livestock is common. Surface sources are more important in the chalklands of southern England, where the widespread cultivation of autumn sown cereals and the absence of hedges combine to increase both the rates of topsoil erosion and the efficiency of sediment transfer to the stream network. Similarly, the pressure of grazing and tourism on the open upland landscapes of the study rivers of northern England and mid Wales encourage sediment production from surface soils, whilst mixed farming promotes soil loss from cultivated and uncultivated areas in the study catchments of north Wales. Further work is required to provide a more rigorous assessment of countrywide variations in sediment sources. However, it is important that the implications of these preliminary findings should be recognised by the authorities and stakeholders responsible for developing strategies for controlling and reducing the degradation of salmonid spawning gravels by siltation. Control of sediment mobilisation from surface sources will clearly require a different approach to control of bank erosion and degradation.
Publisher: Environment Agency
Subject Keywords: Spawning; Sediment; Trout; Salmon; Gravels; Redds; Siltation; Land use.
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