ABSTRACT
Sandy Creek watershed is approximately 70 km west of Austin, Texas. Sixteen percent of the drainage area (now 1,025 km2) of Sandy Creek was added suddenly by the stream capture of an adjacent drainage. The main channel of Sandy Creek as well as that of the capturing tributary (Crabapple Creek) adjusted dramatically as channel and sediment responses were relayed through the system.
Today the channel of Sandy Creek fails to conform to the usual hydraulic geometry relations. In a downstream direction channel-bed width and channelfill depth decrease while flood depth increases anomalously, and gradient, grain size, and sorting remain nearly constant.
Sandy Creek fails to conform to the usual relationships of hydraulic geometry because it has aggraded from upstream. The aggradation is due to successive responses to 1) stream capture of limestone drainage, which abruptly increased sediment-deficient discharges of water, producing 2) degradation in Sandy and Crabapple Creeks, producing 3) rejuvenation of their tributary networks, producing 4) aggradation due to increased sediment loads from the rejuvenated, dominantly granitic drainages.
Investigation of the controls of pre-capture bedrock channel and sediment characteristics revealed that the type and abundance of watershed bedrock exert the most important influence on the adjustments. A spectrum of bedrock channel types was documented. Channels dominated by limestone or schist bedrock characteristically have the thinnest, worst-sorted, and coarsest channel-bed sediments, the lowest width-depth ratios, and pool-riffle thalweg patterns. Channels dominated by granite characteristically have the best sorted and thickest sediments, the highest width-depth ratios, and braided thalweg patterns at low flow. Channels dominated by gneiss or sandstone are transitional.
This spectrum is exemplified by six major channels tributary to Sandy Creek. They exhibit a trend, from east to west, of suspended-load to bedload bedrock channels. Commensurate with the trend is a decrease in granite and gneiss abundance, described quantitatively by their percentages of watershed area. The spectrum of channel types is analogous to that for alluvial channels.
