LITHOLOGIC AND HYDROLOGIC CONTROLS OF DOWNSTREAM SEDIMENTARY CHARACTERISTICS IN THE LLANO RIVER WATERSHED, CENTRAL TEXAS, USA

This study analyses downstream patterns of sediment size and composition for channel-bed material, bars, and banks in the Llano River watershed (11,568km²), a lithologically complex and flash-flood prone fluvial system, in central Texas, USA. Field, laboratory, and statistical analyses and standard sedimentary indices (d₁₆, d₅₀, d₈₄, sorting) were computed at 15 sites along main-stem channels. Channel-bar deposits are characterized by a downstream decrease in particle size, but continuously submerged low-flow-channel deposits have a substantially weaker trend, a discrepancy possibly attributed to uniformity and continuity of hydraulic sorting mechanisms during high flows. Channel-bar deposits also reveal an abrupt downstream reduction in gravel size in the upper watershed, which is attributed to an increase in drainage area. Additionally, an abrupt gravel-to-sand transition occurs immediately downstream of a distinct lithologic change from mostly carbonate rocks to igneous and metamorphic rocks. The downstream decrease in channel-bar particle size occurs despite an increasingly confined alluvial valley, commonly associated with greater unit stream power and coarser sediment. Contrasting with channel-bed material, channel-bank particle size increases downstream because of sand-sized additions from igneous and metamorphic rocks. The consideration of distinctive sedimentological components of a dynamic fluvial system represents a more comprehensive and nuanced study of the topic of downstream sediment trends than prior studies, which is important to a range of engineering, biological, and planning issues at the watershed scale.