137Cs Results and Interpretation of Cesium Soil Data on the Upper Fortymile Wash Alluvial Fan, Amargosa Valley, Nevada.

Studies using 137Cs were used to produce soil Cs profiles and to use them to determine erosion rates on interchannel divides of the Fortymile Wash alluvial fan over the last 50 years. Sample locations whose 137Cs profiles most resemble the reference-sample (stable surface) profiles are located on interchannel divide areas between distributary channels. These profiles are similar to the reference profiles that have low 137Cs values (in the range of 0.02 to 0.08 pCi/g) in the 3 to 6 cm layers. However, the surface layers (1-3 cm depth) typically have values much less than the reference samples from equivalent depths (range from 0.251 to 0.421 pCi/g). The data indicate that many of these interchannel divide areas have had part of the upper layer removed. Interchannel divide areas have the least likelihood of having been submerged during floods over the last fifty years. Thus, the loss of material from these otherwise stable surfaces appears to be due to eolian processes. Erosion of an interchannel divide area with little evidence of recent water movement is most easily explained by eolian removal. Evidence for wind erosion as the predominant process on the interchannel divide areas includes the lack of new or developing stream channels and the presence of modern coppice dunes near channels on interchannel divides. The presence of nearby Big Dune and other eolian deposits provides strong support for eolian erosion and transport. The amount of material removed from the interchannel divide areas was estimated by comparing the 137Cs value of the upper 3 cm layer to that of the reference value and calculating the thickness of the layer that would have to be removed to obtain the lower value. Applying this method across the interchannel divide sample locations indicates 1 to 2 cm of material has been removed from the interchannel divide surfaces in the last 50 years. This results in erosion rates that range from 0.02 to 0.04 cm/yr. These rates are similar to erosion rates of: (a) 0.019 cm/yr predicted to occur on farmland in Amargosa Valley (obtained from BSC 2004 [DIRS 169459], Section 6.4.2 by converting 0.19 kg/m2-yr using ash bulk density of 1 g/cm3; (b) 0.02 cm/yr estimated by the U.S. Department of Agriculture to have occurred on non-cultivated cropland and pastureland in Nevada (obtained from USDA 2000 [160548], Table 11 and calculated using 1 ton/acre-yr x 907 kg/ton x 2.47 x 10-4 acre/m x 0.001 m3/kg [bulk density] x 100 cm/m = 0.02 cm/yr). Overbank deposits on the interchannel divide areas indicative of periodic flooding are uncommon and restricted to narrow strips along the channel banks. The overbank and channel deposit samples have similar 137Cs signatures (the 3 to 6 cm layers and the 6 to 9 cm layers have nearly the same values in the 0.100-0.200 pCi/g range), indicating that the material from each environ was mixed during transport and deposited as a homogeneous sediment. The absence of many overbank deposits along the channel margins today indicates that flows sufficient to form extensive overbank flooding down Fortymile Wash and its distributary channels have not occurred in more than 50 years. Therefore, the channels currently transport most of their sediment load across the fan until it reaches the toe of the fan, where deposition occurs on the broad flats to the south or into the channel of the Amargosa River.