Magnetic tracing of fine-sediment over pool-riffle morphology David J. Milan a, ⁎, Andrew R.G. Large b a Department of Geography, Environment and Earth Sciences, University of Hull, Hull HU6 7RX, UK b School of Geography and Politics, Daysh Building, Newcastle University, Newcastle upon Tyne NE1 7RU, UK abstract article info Article history: Received 26 March 2013 Received in revised form 15 October 2013 Accepted 5 November 2013 Keywords: Magnetic susceptibility Tracer Sediment-transport Pool-riffle Siltation Field studies documenting fine-sediment (b 2 mm) transport in gravel-bed rivers are rare. For the first time in a fluvial environment, a technique that enhances the magnetic susceptibility of sand is used to trace its longitudinal dispersion and storage. This paper describes the methodology behind the artificial magnetic enhancement of iron-stained sand, and presents the results from sand tracing exercises conducted on two gravel-bed channels with pool-riffle morphology; one unregulated and sinuous in nature (site A), the other regulated and straight (site B), both situated on the River Rede Northumberland, UK. Two tonnes of magnetically enhanced tracer sand was introduced to site A and four tonnes to site B, to provide information on fine-sediment storage dynamics, interaction of fines with the stream bed, and rates of movement, expressed as virtual velocity (V i ). Sand transport pathways appeared to differ between the reaches; for site A, sand storage was found on bars and riffle margins with no storage or signs of transport through pools, and in contrast pool storage of tracer was a key feature shown at site B. Topographic forcing may cause differences in sediment sorting at site A; topo- graphic highs tend to have low sand transport rates with sand grains becoming congested in these areas, whereas topographic lows show higher transport rates resulting in greater dispersion. Supply limitation of sand on the falling limb of the hydrograph may also become an issue in the topographic lows at this site. Hydrograph differ- ences between the regulated and unregulated reaches could also play a role; however this could not be quantified in this study. There was no evidence of sand infiltration into the bed at site A; however marginal evidence for infiltration into the near-surface (0–15 cm) substrate voids was found at site B. The general lack of evidence for significant infiltration may reflect limited availability of void space in substrate framework gravels. Tracer sand was transported over the bed surface, with little vertical interaction with the substrate, despite periods of gravel mobilisation at site A. V i over the study duration for site A was 2.28 m day -1 , and 0.28 m day -1 for site B. These values are greater than those calculated using existing predictive equations developed from gravel tracer data, possibly reflecting differences in the mode of transport between bedload and saltation load. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Information regarding fine bedload (b 2 mm) transport in rivers is limited (e.g., Church et al., 1991), despite the significance of this grain- size class to the total sediment load, to instream biota such as salmonids, and macroinvertebrates (Jones et al., 2011; Kondolf et al., 2008; Milan et al., 2000), and its association with toxic heavy minerals in contaminat- ed river systems (Petts et al., 1989). Sand is predominantly transported as the saltation component of the bedload (Garde and Ranga Rangu, 1977), and its transport is complex due to its interaction with bed morphology and the gravel component of the bed substrate. 1.1. Fine-sediments and pool-riffle morphology In gravel-bed streams displaying pool-riffle morphology, its longitudi- nal dispersion has been linked with tractive force variability over the flow regime (de Almeida and Rodríguez, 2012; Jackson and Beschta, 1982; Lisle, 1979; MacVicar and Roy, 2011). At low flow, fines may be stored surficially in areas of low tractive force such as pool exit slopes, channel margins, and in the lee of coarse clasts (Carling and Reader, 1982; Lisle and Hilton, 1992, 1999). Fines may also be stored in void spaces between framework clasts in the sub-surface sediments beneath the armour (Carling and Reader, 1982; Milan et al., 2000). On the rising limb of a flood, tractive force increases over both riffles and pools, and may flush surficial deposits stored in pools (Lisle, 1979). At higher discharges ap- proaching bankfull, the armour layer on the riffles is mobilised, releasing the substrate framework gravel and interstitial fines, increasing sediment-transport rates (Reid et al., 1985). The rate of tractive force increase with discharge has been reported as being greater for pools com- pared with riffles and can equal or exceed adjacent riffles (Keller, 1971; Milan et al., 2001), leading to pool scour and riffle aggradation (de Almeida and Rodríguez, 2012; Vetter, 2011). On the falling limb of a flood, gravels are deposited initially, and then fines may be selectively transported across the bed surface and deposited in areas of low tractive force (e.g. pool exit slopes) (Lisle and Hilton, 1992, 1999; Vetter, 2011). Catena 115 (2014) 134–149 ⁎ Corresponding author. Tel.: +44 1482 465570; fax: +44 1482 466340. E-mail address: d.milan@hull.ac.uk (D.J. Milan). 0341-8162/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catena.2013.11.003 Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena