ABSTRACT
In central Texas the morphology of the streams incised into the Cretaceous limestone bedrock of the Edwards Plateau is controlled by catastrophic floods. This is partly a response to a climatic regime which has produced near record intensity rainfalls of up to 24 hours duration. Upstream from the Balcones Escarpment the high relief, thin lithosols, and sparse vegetation enhance the hydrologic response of the drainage basins. Flooding on two small streams in 1972, resulting from 406 mm of rainfall in four hours, produced spectacular erosion and transport of limestone bedrock. Formation of scour holes and deposition of large gravel bars significantly altered the morphology of the streams. Radiocarbon dates of buried floodplain sediments indicate a minimum recurrence interval of 400 years for geomorphically significant flooding on these streams.
Based on the lack of erosion during floods at the channelfull stage it is proposed that the formation of valley meanders is controlled by floods which greatly exceed the channelfull stage. This is substantiated by the erosion during floods which fill the entire valley and by the statistical correlation of valley meander wavelength with the maximum discharge of record.190 Floodplain formation does not prohibit the formation of valley meanders. Rather, on streams incised into bedrock discontinuous floodplains really are gravel bars deposited and eroded during extremely large floods.
Streams flowing on granitic rocks in the Llano region and on coastal plain sediments downstream fromthe Balcones Escarpment have a morphology adjusted to frequent low-magnitude discharges. This is a function of smaller source rock sediment size, lower relief, and the increased vegetation on more permeable soils causing a reduction in the hydrologic response of the drainage basins. This indicates that, although climate may provide the potential for catastrophic stream behavior, geologic controls may have an overriding influence.
