Abstract
A graded river conveys its sediment load without net deposition or net erosion. A correct understanding of grade is fundamental in genetic stratigraphy, because it represents the critical condition that discriminates between aggradational and degradational regimes in river systems. Conventional wisdom regarding fluvial grade in the alluvial lower reaches of a river emptying into the sea is that global (or large-scale) grade controls the final stable state of alluvial systems, representing an equilibrium response to steady external forcing (i.e., stationary sea level). However, results of recent experimental and numerical modeling of alluvial-deltaic depositional systems conducted by the author’s research group have shown that (1) the alluvial lower reaches of a river are capable of attaining grade only under conditions of falling sea level (not sea level standstill); (2) for grade to be attained and sustained, the falling sea level must follow a particular pattern that depends in part upon the alluvial and subaqueous basal slopes (α and φ respectively); and (3) there exist two different kinds of grade: ‘allogenic grade’ attained by the non-equilibrium response to a decelerating fall in sea level (α < φ), and ‘autogenic grade’ attained via the equilibrium response to a steady fall in sea level at any constant rate (α = φ). Where α > φ, the river never attains grade but continues to aggrade under all conditions of relative sea-level fall. The next generation of genetic stratigraphy models, particularly of alluvial-deltaic depositional systems, should take into account this new view of fluvial grade.
