Variability in fluvially‐dominated, fine‐grained, shallow‐water deltas

Fluvially-dominated, fine-grained, shallow-water deltas are more variable than is generally recognized. Studies of the Mississippi River imply rather persistent river-mouth jet dynamics, resulting in either progressive channel bifurcation around middle-ground mouth bars on lobate deltas, or steady channel progradation to form elongate deltas akin to the modern Mississippi Delta. By contrast, satellite imagery and historical maps of less well-known fluvially-dominated deltas show diverse river-mouth deposits, with plan-form shapes ranging from river-mouth fans, through splay, triangular, frond and tongue-shaped mouth bars, to elongate channels with prominent subaqueous levées. Critically, such deposits may vary along individual channels and across or between individual lobes suggesting that jet variability can be normal, and that a spectrum of jets forms a suite of stable river-mouth deposits. Channel elongation can also be common. In addition to high-inertia jets and delta-head or backwater-mediated avulsion, two further mechanisms are recognized to form elongate channels. Following channel splitting, dominant splits can shoulder aside mouth bars, so that subordinate splits wither and dominant splits extend the parent channel. Alternatively, river-mouth fans, subordinate distributary networks and lobes may be abandoned, so that a dominant parent distributary is rejuvenated by receiving an increase in discharge, allowing progradation to continue. Individual real-world deltas are expected to be characterized by a range of river-mouth deposits, both laterally and over time. However, such variable river-mouth deposits, and different modes of channel elongation, are not generally considered in models of fluvially-dominated deltas – omissions that may have significant impact for land remediation projects on modern deltas, and descriptions of the subsurface constructed to aid resource extraction.