The key role of crevasse splays in prograding river systems: Analysis of evolving floodplain accommodation and its implications for architecture and reservoir potential

Abstract

A generic life cycle applies to crevasse splays in non-degradational fluvial systems, typically ending in healing and abandonment. Crevasse-splay channels adjust to a graded equilibrium profile through proximal erosion and distal deposition, with their distal termini acting as a (prograding) local base level. When proximal incision advances to below the maximum flooding level, a reflux of floodwater occurs during the waning stage of flooding. The resultant decrease in gradient ultimately leads to the backfilling and abandonment of a crevasse splay, provided that the elevation at its distal fringe remains higher than that of the trunk channel floor. Consecutive crevasse splays form an alluvial ridge through lateral amalgamation and subsequent vertical stacking, perching the active river above the surrounding floodplain. Superelevation of the channel thalweg above the distal termini of a prograding crevasse splay leads to avulsion. A high-resolution morphological reconstruction of both the active (and recently abandoned) river(s) and the surrounding floodplain has been established to test the proposed life cycle of crevasse splays and evaluate its role in autogenic avulsion and organisation of the fluvial system. An avulsion can only occur when an overbank path of steepest descent reaches the system base level in a shorter distance (which may partially reuse the existing channel or remnant channel depressions) than the along-channel distance to its terminus. Crevasse splays prograde along this overbank flow path and capture an increasing portion of the total discharge, accelerating their development. When the crevasse apex incises down to or below its trunk channel thalweg, the avulsion is complete. The overbank path of steepest descent (i.e., avulsion path) is governed by floodplain topography, which is largely formed of abandoned alluvial ridges. This leads to compensational stacking of successive prograding channel belts, resulting in fan of amalgamated ridges.