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

Abstract

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.