HR: 1340h
AN: H53C-1278    [Abstracts]
TI: Fluvial Grade Substantiated in Flume Experiments
AU: * Muto, T
EM: tmuto@net.nagasaki-u.ac.jp
AF: Nagasaki University, Faculty of Environmental Studies, 1-14 Bunkyomachi, Nagasaki, 852-8521 Japan
AU: Swenson, J B
EM: jswenso2@d.umn.edu
AF: University of Minnesota at Duluth, Department of Geological Sciences, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 United States
AB: A river which conveys sediment downstream without net deposition or erosion through a series of reaches is referred to as being O`gradedO_L or in equilibrium. This fluvial grade concept, from Gilbert (1877), has long been a subject of debate. It is very uncertain if such a graded river can exist in nature and how it could be recognized presently or in ancient strata. This issue has survived centuries of discussion. We now challenge the classical view, by means of theoretical modeling and flume experiments, and suggest a new view of the nature of graded streams. The topographic setting assumed here is of a river delta accreting across a uniform slope of shelf. Theoretical considerations suggest that the alluvial slope attains and sustains a graded condition grade during sea-level fall, and when the rate of fall reaches a particular pattern of deceleration. This prediction was proved with flume experiments in which two-dimensional deltas were constructed. The agreement between the theory and the experimental results suggests the following view of fluvial grade. Fluvial grade is attained only during sea-level fall and sustained only with a particular pattern of decelerating fall. The graded alluvial slope, characterized by a straight profile, is intrinsically unstable under steady basin forcing and does not represent any final stable stage of the river system. Alluvial aggradation (i.e., out of grade) can proceed as long as sea level remains stationary. The conventional notion of graded conditions equating with the stable sea level or base level is therefore incorrect. This has a number of important implications for stratigraphy.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting