Hillslope failure, river channel widening, and/or construction activity may mobilize sediment from deeper (i.e., meters) sources. Aeolian deposition may be a third source, although
no evidence supports aeolian deposition as a significant source to the rivers studied here. The relative contributions from these sources may change both temporally and spatially in a river. These changes allow only limited selleck compound conclusions to be drawn from a single data point, limiting the success of a mitigation effort that is applied uniformly across a watershed. Contemporary sediment sources are frequently augmented and supplemented by legacy sediment. Legacy sediment comes from anthropogenic sources and activities, such as disturbances in land use/cover and/or surficial processes (James, 2013). For rivers, legacy sediments can originate from incised floodplains (Walter and Merritts, 2008), impoundments behind dams (Merritts et al., 2011), increased hillslope erosion due to historic deforestation (DeRose et al., 1993 and Jennings et al., 2003), and anthropogenic activities
such as construction SRT1720 solubility dmso and land use changes (Wolman and Schick, 1967 and Croke et al., 2001). Legacy sediment can also deliver high loads of contaminants to river systems (Cave et al., 2005 and Lecce et al., 2008). The current supply of sediment is high (Hooke, 2000), as humans are one of the greatest current geomorphic agents. Consequently, combining legacy sediment with increased anthropogenic geomorphic activity makes it even more important to identify the source of sediments in rivers. Sediment sources can be distinguished selleck using the radionuclides lead-210 (210Pb) and cesium-137 (137Cs). 210Pb is a naturally-occurring isotope resulting from the decay of 238Uranium in rock to eventually 222Radon. This gas diffuses into the atmosphere and decays into excess 210Pb, which eventually settles to the ground. This diffusion process creates a fairly consistent level of excess 210Pb in
the atmosphere and minimizes local differences that exist in the production of radon. Rain and settling can subsequently result in the deposition of excess 210Pb, with a half-life of 22.3 years. This atmospheric deposition of excess 210Pb, is added to the background levels that originate from the decay of radon in the soil. “Excess” atmospheric 210Pb occurs because, if the material (in this case the sediment) is isolated from the source (i.e., the atmosphere), this level will decay and decrease in activity. As this excess 210Pb is then correlated with the time of surficial exposure, it is commonly used as a sediment tracer (e.g., D’Haen et al., 2012, Foster et al., 2007, Whiting et al., 2005 and Matisoff et al., 2002). 137Cs is also used as a sediment tracer, although its source is different. It is the byproduct of nuclear fission through reactors and weapon activities, and is not naturally found in the world.