HR: 0800h
AN: H51D-0396    [Abstracts]
TI: Aggradation of Leveed Channels and Their Flood Plains in Arroyo Bottoms
AU: * Vincent, K R
EM: kvincent@usgs.gov
AF: US Geological Survey, 3215 Marine Street, Boulder, CO 80303 United States
AB: Many arroyos that formed by incision more than a century ago in the southwestern United States are currently filling with sediment. This reversal of processes is important because it causes changes in riparian ecology, erosion hazards, ground water recharge, and sediment supply to downstream. Along the Rio Puerco and Chaco Wash in New Mexico, we examined the geometry and facies of channel and floodplain stratigraphy exposed in trenches, used high-resolution dating of the sedimentary beds, and used photographs and other historical evidence to investigate the processes of aggradation in naturally leveed channels within arroyos. Prior to the onset of aggradation, the streambeds were composed of sand and had low relief, and arroyo walls retreated rapidly due to stream undermining. Aggradation began with the formation of sand levees at the margins of the streambeds, followed by formation of newer levees increasingly closer to the thalweg. These levees coincide with rows of woody shrubs (tamarisk and willow), plants that germinated in moist sand along the high-water marks of moderate flows, and survived because subsequent periods lacked flows large enough to remove them. Flow entering a row of woody shrubs decelerates, promoting deposition of suspended sand. Stream flows in this setting are always turbid but do not have the rheology of debris flows. The rows of shrubs probably are a requirement for initial formation of sand levees on low relief streambeds in this setting. As new levees formed closer to the thalweg the channel effectively narrowed, and smaller discharges overtopped the levees adjacent to the channel. Those closer levees accumulated sand most rapidly, leaving the suspended sand concentration depleted by the time water reached more distant ones. All levees aggraded vertically. As the main channel narrowed it acquired a roughly trapezoidal-shape (Top Width/Depth ~ 9 to 5) with banks inclined close to the angle of repose. In addition, sediment deposited on the emerging flood plain became dominated by silt (or clay) while the levees next to the channel remained dominated by fine or very fine sand. Furthermore, the channel and floodplain aggraded at similar rates and thus had come into geomorphic equilibrium. Vertical accretion of the channel banks, which are the flanks of channel-margin levees, was accomplished by deposition of inclined lamina and very thin beds dominated by silt that have fairly uniform thickness. This may have been promoted by rapid infiltration of stream water into the banks, filtering fine suspended sediment at the solid interface.
DE: 0483 Riparian systems (0744, 1856)
DE: 1160 Planetary and lunar geochronology
DE: 1825 Geomorphology: fluvial (1625)
DE: 1861 Sedimentation (4863)
SC: Hydrology [H]
MN: Fall Meeting 2005