THE GUN BARREL HYPOTHESIS FOR DEFAULT DELTA FORMATION

Deltas are characteristically known to have triangular/delta morphology owing to distributary bifurcation. Bifurcation is usually attributed to accumulation of sand at the delta mouth. Numerous studies of modern deltas reveal a wide degree of variation in delta form attributed to modifications of the distributary process and delta-front sand accumulation by basinal forcing factors (e.g., tides, waves, etc.). Fluvially dominated deltas that experience minimal basinal forcing are known to develop minimal bifurcation and, thus, tend to be the most linear in form.

Examination of deltas formed in shallow freshwater lakes, where basin factors are minimal, reveal that they typically form linear prograding channels that stretch without bifurcation for lengths up to two orders of magnitude beyond channel width. Field examination finds sand to be minimal within the distal channel and mouth of these deltas. Linear delta form is here attributed to partitioning of fine-grained suspended load toward the basin owing to faster transport rates of these sediments compared to bed load. The delta form is this dominated by construction forms favored by mud deposition, even in otherwise bed load-dominated rivers. Suspended load tends to form subaqueous levees that build to emergence before the lagging bed load arrives and tends to develop a well-encased open channel. These levees become further hardened by vegetation. This leveed channel forms a hardened ‘gun barrel’ that is not breached by blockage from the later-arriving sand, but rather projects these sediments down channel during floods or forms maintained channel bars. These linear deltas will form in most all cases unless the ‘gun barrel’ is shortened by other basin processes. In these cases the sand is able to accumulate at the channel mouth and drive bifurcation/scatter of sediments. Marine systems where levees are commonly destroyed by waves and tides, or the gun barrel is clogged, thus tend to have deltaic forms. Deltas otherwise are linear by default.