3. Fallout Radionuclides

A number of short-lived radioactive isotopes of both natural and anthropogenic origins which are (or were) atmospherically deposited over the landscape have been extensively utilized as sediment tracers in riverine environments. The three most extensively utilized isotopes, which are often referred to as fallout radionuclides (or FRNs), include in decreasing order of application, \({{}^{137}\mathrm{Cs}},\,{{}^{210}\mathrm{Pb}_\mathrm{ex}}\), and \({{}^7\mathrm{Be}}\). Herein we examine the primary ways in which these three isotopes have been applied to gain insights into the riverine sediment system. More specifically, we explore the strengths and weaknesses of using FRNs in combination with mixing models to determine sediment provenance at the catchment scale, particularly with regards to determining whether the sediment was derived by means of sheet, rill, gully, or bank erosion. The nuclide inventory approach is also examined for its ability to characterize other components of the sediment system at much smaller spatial scales, including the redistribution of sediment on hillslopes and between landscape units. Our discussion concludes by examining the ability of \({{}^7\mathrm{Be}}\) to document dynamic processes operating along the channel bed by determining sediment residence times, scour and fill depths, particle filtration, and sediment travel distances.