RUI: Fluvial Terraces: A Tool for Integrating Geomorphic Processes, Climatic and Tectonic Events, and Landscape Development

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This is a geomorphic investigation of fluvial processes in an area in which tectonic activity varies between river basins, and climate varies with time among the basins. Tectonic controls will dominate terrace formation in high uplift rate regions, and climatic controls will gain increasing importance as uplift rate declines. We propose to test this hypothesis on the basis of differences and similarities in the terrace sequences of three moderate size rivers developed in argillaceous sandstone in coastal northern California. Present climatic controls on amount and intensity of precipitation and temperature are very similar for each drainage basin, although these parameters have varied with late Cenozoic climatic perturbations that ultimately drove the rise and fall of eustatic sea level. Base-level processes related to relative rise of the land mass vary by an order of magnitude from the southernmost to the northernmost rivers. Fieldwork in each watershed will consist of continuous surveying and mapping of the longitudinal profiles of erosional terraces, depositional terraces, and the modern channel and floodplain. Seismic refraction studies will quantify depth of fill where not observable. The chronology of terrace formation will be constrained by radiometric dating of organic material from alluvial sediments either resting upon erosional terraces, or within deposits that comprise depositional terraces. This work will allow quantification of the nature and rates of propagation of channel incision and valley in-filling in response to changing climate and long-term tectonism. Comparison of the results for the three rivers, and thus separation of tectonic and climatic controls, should allow: 1) better understanding of the nature of fluvial response to change, 2) interpreting the environmental significance of strath versus fill terraces, and of preserved flights of terraces in general, and 3) integrating an understanding of geomorphic processes with models of landscape evolution.