The application of optical dating methods to late Quaternary alluvial environments in the western United States

This dissertation project used optical dating techniques to assess the impacts of changing climatic conditions on three alluvial systems in the western United States. The first study compared the results of optical (both multiple and single-grain) and radiocarbon dating techniques in ephemeral stream deposits in western Nebraska. The single-grain technique provided the estimates that best agreed with relative age controls, while the radiocarbon and multiple-grain techniques yielded age over-estimations of up to ∼ 750 and 1,500 years, respectively. In the second study optical dating was used to evaluate the genesis of fluvial terraces along Wyoming's Laramie Range. Previous studies from the region argued that fluvial incision rates are governed by sediment availability where the high sediment loads during glacial periods caused a reduction in stream incision rates. However, the optical chronology from this study indicates that incision rates were greatest during the last glacial period and lowest during the Holocene when stream sediment loads were reduced. These findings suggest that terraces were more likely generated as a result of changing stream power where the higher flood magnitudes during the glacial periods resulted in increased incision rates. The last study applied optical dating to alluvial fan deposits in the Sonoran Desert of Arizona and California. Previous work in the region had suggested that fans were stabilized from ∼ 24-14 ka and that sedimentation rates increased from ∼ 14-9 ka when reductions in vegetation densities left hillslopes vulnerable to erosion. However, this study showed that most of the fans in the study area aggraded between ∼ 28-16 ka when changes in fan deposition rates cannot be attributed to changes in vegetation density. Greater deposition rates during this period correspond with greater regional precipitation levels suggesting that fan sedimentation rates are higher when precipitation levels are elevated. Overall, optical dating techniques provided reliable age control for alluvial deposits in each of these settings. The results of the latter two studies suggest that changes in stream power can have significant effects on stream incision rates and alluvial fan deposition rates.