Precambnan outcrops in the Llano Uplift of central Texas furnish the Colorado River with a distinctive granitic load which changes systematically as it moves downstream. In 160 miles between Austin and the Columbus-Eagle Lake area, coarse granitic gravel diminishes in size by about 50 percent and changes from a dommantly granite-gneissaplite assemblage to a dominantly pegmatite-graphic granite assemblage. Coarse quartz and chert gravel diminish in size by about 30 percent and 20 percent, respectively. In this reach, the Colorado flows on a flood plain whose alluvial width and depth stay fairly constant, a situation unfavorable for significant down-valley sorting (Mackin, 1963). Field observations generally lead to the same conclusion.
Abrasion and its relationship to weathering was studied by running fresh and weathered specimens of Colorado River granitic gravel in a Kuenen-type abrasion tank at the University of Texas. Degree of weathering was slight to moderate, with biotite and feldspar the primary and secondary targets. Abrasion of fresh granitic gravel during 160 miles of travel produced only a 10 percent reduction in size. Abrasion of weathered gravel produced a reduction of more than 50 percent and was clearly related to lithology; the biotite-bearing rocks (granite, gneiss, and some aplite) were the least durable, the biotite-free rocks (pegmatite and graphic granite) most durable. Abrasion of quartz and chert produced a reduction in size of less than 10 percent.
Changes in size and lithology of coarse granitic gravel along the lower Colorado River are best explained by abrasion of particles tha t weather slightly during periods of temporary alluvial storage. Reduction in size of quartz and chert are not satisfactorily explained for the reach between Austin and Columbus-Eagle Lake, but downstream from Eagle Lake, sorting appears to be in control.