A multi-proxy assessment of terrace formation in the lower Trinity River valley, Texas

Abstract. A proposed null hypothesis for fluvial terrace formation is that internally generated or autogenic processes such as lateral migration and river-bend cutoff produce variabilities in channel incision that lead to the abandonment of floodplain segments as terraces. Alternatively, fluvial terraces have the potential to record past environmental changes from external forcings that include temporal changes in sea-level and hydroclimate. Terraces in the Trinity River valley have been previously characterized as Deweyville groups and interpreted to record episodic cut and fill during late Pleistocene sea-level variations. Our study uses high-resolution topography of a bare-earth digital elevation model derived from airborne lidar surveys along ~88 linear km of the modern river valley. We measure both differences in terrace elevations and widths of paleo-channels preserved on these terraces in order to have two independent constraints on terrace formation mechanisms. For 52 distinct terraces, we quantify whether there is a clustering of terrace elevations – expected for allogenic terrace formation tied to punctuated sea-level and/or hydroclimate change – by comparing variability in a chosen set of terrace elevations against variability associated with randomly selected terrace sets. Results show Deweyville groups record an initial valley floor abandoning driven by allogenic forcing, which transitions into autogenic forcing for the formation of younger terraces. For 79 paleo-channel segments preserved on these terraces, we connected observed changes in paleo-channel widths to estimates for river paleo-hydrology over time. Our measurements suggest the discharge of the Trinity River has changed systematically by a factor of ~2 during the late Pleistocene. Methods introduced here combine river-reach scale observations of terrace sets and paleohydrology with local observations of adjacent terrace-elevation change and paleo-channel bend number to show how interpretations of allogenic versus autogenic terrace formation can be evaluated within a single river system.