1. Introduction

The contamination of surface waters with heavy metals (elements with a specific density N 5 g cm−3 ; Callender, 2003) is a major environmental concern (Driscoll et al., 2013; Sutherland, 2000). Heavy metals exhibit a range of toxic effects to aquatic and terrestrial biota (Driscoll et al., 2013; Flemming and Trevors, 1989; Mebane, 2010) in addition to impacts on human health (Lanphear et al., 2005; Tchounwou et al., 2012). Although natural sources exist (e.g., weathering and wildfires), anthropogenic activities (e.g., mining and smelting, urbanization and industrial processes) substantially increase heavy metal fluxes to rivers (Horowitz and Stephens, 2008; Horowitz et al., 2012; Macklin et al., 2006; Sutherland, 2000). Metals in aquatic environments are conventionally classified as particulate or dissolved with dissolved forms operationally defined as material passing through a 0.45 μm filter (Nystrand et al., 2012; Owens et al., 2005). While dissolved forms are considered immediately bioavailable, the extent to which particulate forms are bioavailable will depend on the lability of metal species in particulate phase and the physicochemical properties of the environment (Eggleton and Thomas, 2004). A large proportion of heavy metal transport to the world's lakes and oceans occurs via rivers (Callender, 2003) and due to the strong affinity of metals for soil/ sediment surfaces (Horowitz, 1991) most of this is associated with the particulate phase (Horowitz et al., 2012; Martin and Meybeck, 1979; Viers et al., 2009). There is now strong evidence for anthropogenic contamination of suspended sediments in many of the world's rivers (Horowitz et al., 2012; Owens et al., 2005; Viers et al., 2009), and erosion events are a major vector for the transport of suspended sediments and associated contaminants from terrestrial to aquatic environments (Horowitz et al., 2012; Rickson, 2014; Sutherland, 2000; Walling, 2005). As such, understanding the role of erosion on heavy metal dynamics in river systems is an active and important area of research.