Late Quaternary geomorphic evolution of the Colorado River, Bastrop and Fayette counties, Texas




Looney, Robert Michael

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Ancient and modern flood plain and channel morphology was mapped in the late Quaternary alluvial valley of the Colorado River between Utley and La Grange, Texas. The analysis of N.A.S.A.-generated color aerial infrared photography, SKYLAB remote sensing imagery, and aerial panchromatic photography revealed nine assemblages of fluvial channel patterns. The paleochannels occur on multiple flood plain and terrace levels and are associated with deposits with variable textures, sedimentary structures, and lithologic composition. Quantitative geomorphic analysis showed that bankfull width for the late Quaternary Colorado River varied from 450 m to 200 m, meander wavelength from 575 m to 1730 m, and sinuosity from 1.3 to 3.6. The channel adjustments from low sinuosity to high sinuosity streams was accompanied by a decrease in meander wavelength and bankfull width. Sedimentological analyses show corresponding changes in grain size from sand and gravel to sand and silt transporting streams. The nine channel assemblages of the Colorado River reflect changes in runoff and sediment load characteristics from upstream catchment areas. These runoff and sediment load changes correlate with an alternating arid-humid climate in central Texas during the late Quaternary. The resulting hypothesis is that channel 7 is pre-Wisconsinan; channels 6, 6A, and 6B are Wisconsinan; channel 5 is a dry period at the beginning of the Holocene; channel 4 is a humid Holocene phase; and channel 3 to the modern channel are the most recent Holocene fluctuations. Sedimentary structures and paleo-hydrologic implications indicate that channels 7 and 6 were laid down by broad, shallow braided streams, Channels 6A and 5 were either braided or coarse meander belt fluvial systems. The small channel width, meander wavelength, and low sinuosity indicates that channels 6B and 4 operated as fine grained meander belt fluvial systems. Channels 3 and 2 are similar to the modern Colorado River which is a bed-load (high bed-load/discharge ratio) stream transporting coarse sand and pebble-to-cobble gravel