HR: 13:55h
AN: H53E-02    [Abstracts]
TI: Evidence for autogenic cyclicity in ephemeral stream cut-fill dynamics
AU: * Tucker, G E
EM: gtucker@cires.colorado.edu
AF: CIRES & Dept of Geological Sciences, University of Colorado 2200 Colorado Ave 399 UCB, Boulder, CO 80309-0399 United States
AU: Arnold, L
EM: lee.arnold@st-peters.oxford.ac.uk
AF: Oxford Luminesence Research Group, School of Geography & the Environment, Oxford, OX1 3TB United Kingdom
AU: Stokes, S
EM: stephen.stokes@geog.ox.ac.uk
AF: Oxford Luminesence Research Group, School of Geography & the Environment, Oxford, OX1 3TB United Kingdom
AB: Cut-fill sequences in ephemeral channels are often attributed to climate forcing. Yet it has been proposed that these sequences can also arise from internal dynamics, without the need for an external trigger. This raises the following question: under what conditions, and by what mechanisms, can such autogenic cycles occur? Intrinsic geomorphic thresholds have been cited as important controls of cut-fill cycle initiation in ephemeral streams across the American southwest, but the types of controlling geomorphic thresholds involved, and the mechanisms leading to the threshold-crossing events, are not clear. Here we use a numerical model to identify necessary and sufficient conditions for autocyclic behavior in ephemeral-channel networks. Model simulations of a hypothetical semi-arid drainage basin demonstrate the existence of alternating steady-state epicycles of aggradation and erosion without any prior changes in independent external variables. The two key intrinsic control mechanisms responsible are (1) threshold channel slope angles, and (2) upstream signal propagation following threshold-crossing events. Analysis of the process-response relationships occurring within the catchment reveals the important environmental and basin conditions that promote autocyclic behaviour, and the sorts of channel-hillslope interactions and feedbacks that are an integral part of this dynamic behavior. The results provide process-based evidence for the existence and importance of intrinsic controls on cut-fill epicycles in ephemeral systems. The implications of these findings are discussed in the context of the arroyo problem and the interpretation of field records.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1848 Networks
DE: 1869 Stochastic processes
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting