Downstream trends of alluvial sediment composition and channel adjustment in the Llano River watershed, Central Texas, USA : the roles of a highly variable flow regime and a complex lithology

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2009-05

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Heitmuller, Franklin Thomas

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Abstract

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.

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