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Autogenic attainment of large-scale alluvial grade with steady sea-level fall: An analog tank-flume experiment

¹ Faculty of Environmental Studies, Nagasaki University, Nagasaki 852-8521, Japan
² Department of Geological Sciences and Large Lakes Observatory, University of Minnesota, Duluth, Minnesota 55812, USA

A graded river conveys its sediment load without net deposition or erosion. The graded state is thought to represent the long-term response of alluvial rivers to steady external forcing. We show here that alluvial rivers building deltas can be in grade as an autogenic response to steady sea-level fall. Consider an antecedent graded river profile, the upstream end of which consists of an alluvial-bedrock transition, and the downstream end of which is a fixed overfall where constant sea level is maintained. The antecedent graded profile is then drowned by a jump in sea level, after which sea level drops. The result is a new river profile ending in a prograding delta that deposits on top of the antecedent profile. If the rate of sea-level fall is constant and the length of the antecedent reach is sufficient, the new profile eventually becomes parallel or quasi-parallel to the antecedent profile, maintaining grade as it progrades. In the experiments reported here, series of graded river profiles with prograding deltas are created by stacking fluviodeltaic systems; each graded profile and its associated delta is stacked on its immediate predecessor. For each fluviodeltaic system, a graded alluvial profile is attained with any constant rate of sea-level fall, provided that the antecedent profile is of sufficient length. Experiments suggest that this autogenic approach to grade is more rapid for higher rates of sea-level fall, lower rates of sediment supply, and higher water discharges.

Key Words: alluvial • equilibrium • experimental studies • grade • rivers • sea level

This article has been cited by other articles:

				A. L. Petter and T. Muto Sustained Alluvial Aggradation and Autogenic Detachment of the Alluvial River from the Shoreline in Response to Steady Fall of Relative Sea Level Journal of Sedimentary Research, February 1, 2008; 78(2): 98 - 111. [Abstract] [Full Text] [PDF]

				T. Muto, R. J. Steel, and J. B. Swenson Autostratigraphy: A Framework Norm for Genetic Stratigraphy Journal of Sedimentary Research, January 1, 2007; 77(1): 2 - 12. [Abstract] [Full Text] [PDF]