Dating Sediment in a Fast Sedimentation Reservoir using Cesium-137 and Lead-210

More than 10,000 reservoirs have been constructed in agricultural water-sheds in the United States since the 1940s to control floods and sediments. Reservoir sedimentation records provide a unique opportunity to retrospectively study the effects of land use changes and climate variations on sediment production if the sediment chronology could be properly determined. The objectives of this study were to: (i) evaluate the applicability of four unsupported Pb-210 (Pb-210(ex)) models for dating deposited sediments in fast-sedimentation environments; (ii) estimate the sedimentation rates; and (iii) relate the sedimentation rates to changes in soil conservation measures, land use, and climate. Three sediment profiles were sampled at 5-cm intervals in a flood control reservoir in the Fort Cobb Reservoir Experimental Watershed in southwestern Oklahoma. Radioactivity was measured with a gamma spectrometer for radioisotopes Cs-137 and Pb-210. Four Pb-210(ex) dating models were tested. Three models (constant flux-constant sedimentation [CFCS], constant initial concentration [CIC], and constant rate of supply [CRS]), which are widely used in the lacustrine and marine environments under slow-sedimentation conditions, were not applicable to fast-sedimentation conditions because of considerable Pb-210(ex) input with the sediment, which is assumed negligible in the models. A modified CRS model that uses Cs-137 age markers as references improved determination of the sediment chronology. The average sedimentation rates were three to five times higher during the 1957 to 1963 time period than during the 1964 to 2011 period. The significant sediment reductions coincided with the implementation of numerous conservation measures in the watershed, including check dams, terraces, changing cropping patterns, and progressive adoption of no-till and conservation tillage systems.