This study investigates the downstream controls of alluvial sediment
composition and river channel adjustment in the Llano River watershed, Central Texas,
USA. The Llano River watershed is characterized by a highly variable, flood-prone
flow regime and a complex lithology of Cretaceous carbonate rock, Paleozoic
sedimentary rock, and Precambrian igneous and metamorphic rock. Sedimentary
variables for this study include particle size, sorting, carbonate content, and magnetic
susceptibility. Channel adjustment includes the planform dimension and cross-sectional
dimensions of bankfull- and macro-channels. Nineteen sites along the Llano River and
selected tributaries were visited to measure cross-sectional channel geometry and
sample bed, bank, and overbank sediment. Laboratory analyses of sediment and
hydraulic analyses of cross sections were accompanied by analyses of partial-duration
flood frequency, flow resistance, hydrography, digital elevation models, and statistical correlation. Findings include: (1) channel-bed material reduces in size with downstream
distance, despite increasing valley confinement and bedrock exposure; (2) the
downstream decrease in particle size is more evident for channel-bar deposits than for
low-flow-channel (thalweg) deposits; (3) an abrupt gravel-to-sand transition occurs
about 20 kilometers downstream of the Paleozoic-Precambrian contact; (4) an abrupt
coarse- to fine-gravel transition occurs between 75 and 90 kilometers downstream the
North Llano and South Llano Rivers; (5) channel-bank material increases downstream,
contrasting with decreases in bed material; (6) carbonate content and magnetic
susceptibility of alluvial sediment are inversely related, with carbonate content peaking
near Junction; (7) four general categories to classify reaches of the North Llano, South
Llano, and Llano Rivers are based on hydrology, planform morphology, lithology, and
valley confinement; (8) mean depth increasingly compensates for bankfull discharge in
a downstream direction; (9) mean depth compensates more than width for macrochannels;
and (10) the return periods for bankfull and macro-channels are about 1 to 2
years and greater than 10 years, respectively. The results of this study will contribute to
fluvial geomorphic theory of downstream trends in sediment composition and channel
adjustment; as well as inform applied efforts related to aquatic biology, flood hazards,
infrastructure design, and riparian and water-resource management in the region. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/6900 |
| Date | 05 February 2010 |
| Creators | Heitmuller, Franklin Thomas |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Format | electronic |
| Rights | Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works. |
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