Abstract: Late Quaternary Geomorphic Evolution of Colorado River, Inner Texas Coastal Plain

R. Michael Looney, Victor R. Baker

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 NASA-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 of variable textures, sedimentary structures, and lithologic composition. Quantitative geomorphic analysis showed that bank-full width for the late Quaternary Colorado River ranged from 450 to 200 m, meander wavelength from 575 to 1,730 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. Sedimentologic analyses show corresponding changes in grain size from streams transporting sand and gravel to those transporting 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 definitely pre-Wisconsinan and probably Sangamonian in age. It contains both arid and humid phases. Channels 6, 6A, and 6B are late Wisconsinan arid and humid phases. Channel 5 appears to represent an arid phase in the early Holocene. The later Holocene includes a himid phase (channel 4) and later arid phases.

Sedimentary structures and preserved channel morphology indicate that channels 7 and 6 were laid down by broad, shallow, braided streams. Channels 6A and 5 were either braided or meandering streams transporting mainly coarse bed load. The small channel width, meander wavelength, and low sinuosity indicate that channels 6B and 4 operated as fine-grained meanderbelt fluvial systems. Channels 3 and 2 are similar to the modern Colorado River which is a bed-load (high bed load to discharge ratio) stream transporting coarse sand, pebbles, and cobbles.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas