LATE QUATERNARY FLOODPLAIN HISTORY OF THE BRAZOS RIVER IN EAST-CENTRAL TEXAS

The floodplain along a 75-km segment of the Brazos River, traversing t he Gulf Coastal Plain of Texas, has a complex late Quaternary history. From 18,000 to 8500 yr B.P., the Brazos River was a competent meander ing stream that migrated from one side of the floodplain to the other, creating a thick layer of coarse-grained lateral accretion deposits. After 8500 yr B.P., the hydrologic regime of the Brazos River changed. The river became an underfit meandering stream that repeatedly became confined within narrow and unstable meander belts that would occasion ally avulse. Avulsion occurred four times; first at 8100 yr B.P., then at 2500 yr B.P., again around 500 yr B.P., and finally around 300 yr B.P. The depositional regime on the floodplain also changed after 8500 yr B.P., with floodplain construction dominated by vertical accretion . Most vertical accretion occurred from 8100 to 4200 yr B.P. and from 2500 to 1250 yr B.P. Two major and three minor periods of soil formati on are documented in the floodplain sequence. The two most developed s oils formed from 4200 to 2500 yr B.P. and from around 1250 to 500 yr B .P. These changes on the floodplain appear to be the result not of a s ingle factor, but of the complex interplay among changes in climate, s ediment yield, and intrinsic floodplain variables over time. (C) 1995 University of Washington.