Adjustment to Quaternary Climatic Change by the Colorado River in Central Texas

Geomorphic mapping of flood plain and channel morphology in the Quaternary alluvial valley of the Colorado River revealed ten cross-cutting assemblages of fluvial channel patterns and associated sedimentary structures and textures. Quantitative studies of the modern and ancient patterns between Austin and La Grange, Texas, indicate systematic changes in channel width, meander wavelength, sinuosity, and sediment type through time. The changes appear to be cyclic, although the amplitudes of the cycles vary. Sufficient morphology is preserved to utilize empirical correlations of meander wavelength to discharge as an aid in paleohydrologic interpretation. However, the influences of sediment load and flood magnitude-frequency changes preclude quantitative speculation. Data from fluvial sediments, relict paleosols, and paleobiological studies support the hypothesis that Quarternary climate in central Texas alternated between relatively arid and humid intervals. Humid phases were characterized by fine-grained loads and high-sinuosity patterns on flood plains. Changes to arid conditions abruptly interrupted flood plain development with degradational activity. Continued arid conditions promoted aggradation by low-sinuosity streams transporting polygenetic suites of coarse sediment derived from nearby, eroding hillslopes. The transition to humid streams was gradual and probably accompanied by more uniform streamflow characteristics.