1 BHS Eleventh National Symposium, Hydrology for a changing world, Dundee 2012. ISBN: 1903741181 © British Hydrological Society Fine sediment sources, sedimentation risk and siltation of salmonid spawning gravels in the Herefordshire Lugg, UK J. Stopps 1* , L.J. McEwen 2 and D.J. Milan 1 1 School of Natural and Social Sciences, University of Gloucestershire, UK 2 Department of Geography and Environmental Management, University of West of England, UK *Email: jasonstopps@connect.glos.ac.uk Context Contemporary applied river research requires an under- standing of ine sediment sources and transport pathways within catchments. Excessive ine sediment pollution of rivers is known to harm the ecological status of surface water and instream biota (Sear et al., 2008). For example, gravel-bed rivers provide important spawning habitat for salmonids but the quality of spawning gravels is particularly sensitive to the changing physical character of rivers and their catchments (Kondolf et al., 2008). The accumulation of interstitial ine sediment is critical for salmon embryo survival (Sear et al., 2008). Research evidence suggests that where ine sediment (<2 mm) exceeds 15% (+/– 5%) of the channel bed material, salmonid embryo survival reduces to less than 50% (Milan et al., 2000). Management of the ine sediment problem therefore requires a catchment-scale approach (e.g. Sear et al., 2009), underpinned by sustainability science that links land and water phases of ine sediment transfer and storage. However, most assessments of ine sediment dynamics have focused at the reach-scale and tend not to have a more holistic catchment-scale focus. This project therefore is establishing the spatial and temporal character of ine in-channel sediment and modelling its sources — an essential evidence base for effective sustainable catchment management. This will allow the successful implementation of mitigation measures to reduce the impact of excessive sediment pollution. Study Area The River Lugg catchment (1070 km²) and its parent, Wye, (4136 km²) are both European candidate Special Areas of Conservation (SAC) under the EU Habitats Directive, with salmonids Atlantic salmon (Salmo salar) and brown / sea trout (Salmo trutta) as key conservation interests. Although the River Lugg provides excellent spawning habitat, there have been dramatic declines in salmon stocks since the 1970s (River Wye Salmon Action Plan, 2003). Degradation of spawning habitat by siltation is regarded as a major contributing factor to reduced salmonid numbers. Previous studies (Theurer et al., 1998; Naden et al., 2003) highlighted the problem across England and Wales, with increasing siltation risk due to catchment land use and poor agricultural practice. As a result, there are site-speciic projects examining the spatial and temporal nature of siltation and its impact on egg/larvae life stages of salmonids and their spawning habitat (Environment Agency R&D Report, 2003; DEFRA project 2009-2012). The River Lugg and its main tributary, the River Arrow, both fail to achieve the EC Freshwater Fish Directive (78/659/EEC) suspended solids (SS) guideline standard of 25 mg/l, with episodic high sediment loadings. Several natural and human-induced factors affect the Lugg’s vulnerability to ine sediment inputs, including easily erodible ine sandy soils overlying Old Red Sandstone bedrock, and increased coupling between the farmed slopes, loodplain and the channel. Recent accelerated diffuse ine sediment pollution Abstract This paper presents initial results of a three-year ine sediment research monitoring programme funded through the SITA Trust’s ‘Enriching Nature’ programme, the Environment Agency and Arup. This research has been evaluating a catchment-wide, monitoring approach to establish spatial and temporal patterns, and sources of ine sediment in the Herefordshire Lugg catchment, UK. The project aims to provide the underpinning science for the implementation of DEFRA’s Catchment Sensitive Farming initiative, promoting ine sediment management in the Lugg catchment. Delivery of suspended sediments to ive sink sites was monitored between April 2009 and April 2012, using time-integrated samplers. Additional lood deposits were also sampled following the larger events. Gravel quality of the bed substrate was also established at each of the sites using freeze core sampling. Geochemical analysis, using ICP-IES, was conducted on all the sink samples (130+) and on 270+ potential source samples taken from tributaries upstream of the sink sites. A two-stage statistical procedure involving a Kruskal-Wallis H test, and Discriminant Function Analysis was undertaken to identify the optimum geochemical ingerprint. This ingerprint was then used to identify sediment sources using a multivariate mixing model. Initial indings are presented for temporal variations in sources for the River Arrow catchment, a key tributary. The approach appears successful at identifying problem sub-catchments (e.g. Honeylake Brook and Curl Brook). Further analysis will investigate ine sediment sources within these problem sub-catchments. doi: 10.7558/bhs.2012.ns50