ABSTRACT

Ancient and modern flood plain and channel morphology was mapped in the late Quaternary alluvial valley of the Colorado River through the Inner Texas Coastal Plain 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 compositions. Quantitative geomorphic analysis showed that bankfull width for the late Quaternary Colorado River varied from 550 m to 200 m, meander wavelength from 5750 m to 1730 m, and sinuosity from 1.3 to 3.6. The channel adjustments from low-sinuosity to high-sinuosity streams were accompanied by a decrease in meander wavelength and bankfull width. Sedimentological analyses show that such adjustments were also accompanied by changes in grain size from sand and gravel to sand and silt.

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 occurred during an alternating arid-humid climate that characterized the late Quaternary of Central Texas. The oldest channel-pattern assemblage (channel 7) is equivalent to deposits of the Capitol Terrace at Austin, Texas. Channels 6, 6A, and 6B are equivalent to deposits of the Sixth Street Terrace and probably represent late Wisconsin arid and humid river phases. Channel 5 appears to represent a relatively arid phase of the Colorado River in the early Holocene. The later Holocene includes a relatively humid phase (4) and later trasitional phases.

Sedimentary structures, preserved channel morphology and lithology-roundness-size relationships indicate that channels 7 and 6 were laid down by broad, shallow-braided streams in an arid climate. Channels 6A and 5 were either braided or meandering streams transporting mainly coarse bedload. The small channel width,low meander wavelength, and high sinuosity of channels 6B and 4 indicate that they operated as fine-grained meander belt fluvial systems in relatively humid climatic periods. 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 from a basin of climatic transition.