title: Understanding the controls on deposited fine sediment in the streams of agricultural catchments.
abstract: Excessive sediment pressure on aquatic habitats is of global concern. A unique dataset, comprising instantaneous measurements of deposited fine sediment in 230 agricultural streams across England and Wales, was analysed in relation to 20 potential explanatory catchment and channel variables. The most effective explanatory variable for the amount of deposited sediment was found to be stream power, calculated for bankfull flow and used to index the capacity of the stream to transport sediment. Both stream power and velocity category were highly significant (p ≪ 0.001), explaining some 57% variation in total fine sediment mass. Modelled sediment pressure, predominantly from agriculture, was marginally significant (p<0.05) and explained a further 1% variation. The relationship was slightly stronger for erosional zones, providing 62% explanation overall. In the case of the deposited surface drape, stream power was again found to be the most effective explanatory variable (p<0.001) but velocity category, baseflow index and modelled sediment pressure were all significant (p<0.01); each provided an additional 2% explanation to an overall 50%. It is suggested that, in general, the study sites were transport-limited and the majority of stream beds were saturated by fine sediment. For sites below saturation, the upper envelope of measured fine sediment mass increased with modelled sediment pressure. The practical implications of these findings are that (i) targets for fine sediment loads need to take into account the ability of streams to transport/retain fine sediment, and (ii) where agricultural mitigation measures are implemented to reduce delivery of sediment, river management to mobilise/remove fines may also be needed in order to effect an improvement in ecological status in cases where streams are already saturated with fines and unlikely to self-cleanse.
authors.affiliation: Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: P S
authors.initials: PS
authors.lastname: Naden
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK.
authors.forename: J F
authors.initials: JF
authors.lastname: Murphy
authors.affiliation: Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: G H
authors.initials: GH
authors.lastname: Old
authors.affiliation: Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: J
authors.initials: J
authors.lastname: Newman
authors.affiliation: Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: P
authors.initials: P
authors.lastname: Scarlett
authors.affiliation: Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: M
authors.initials: M
authors.lastname: Harman
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK.
authors.forename: C P
authors.initials: CP
authors.lastname: Duerdoth
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK.
authors.forename: A
authors.initials: A
authors.lastname: Hawczak
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK.
authors.forename: J L
authors.initials: JL
authors.lastname: Pretty
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK.
authors.forename: A
authors.initials: A
authors.lastname: Arnold
authors.affiliation: Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: C
authors.initials: C
authors.lastname: Laizé
authors.affiliation: Geography and Environment, University of Southampton, Southampton SO17 1BJ, UK.
authors.forename: D D
authors.initials: DD
authors.lastname: Hornby
authors.affiliation: Geography and Environment, University of Southampton, Southampton SO17 1BJ, UK; Sustainable Soils and Grassland Systems Department, Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK.
authors.forename: A L
authors.initials: AL
authors.lastname: Collins
authors.affiliation: Sustainable Soils and Grassland Systems Department, Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK.
authors.forename: D A
authors.initials: DA
authors.lastname: Sear
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK.
authors.forename: J I
authors.initials: JI
authors.lastname: Jones
keywords: Agricultural sediment pressure; Agricultural streams; Channel substrate; Deposited fine sediment; Saturated fine sediment fraction; Stream power