FLUVIO-LACUSTRINE VS. FLUVIO-MARINE AND THE RARITY OF THE CLASSIC DELTA IN THE MODERN LAKE

The term delta was coined by Herodotus in days B.C. for the characteristic “delta” shape of the Nile where it met the sea. Since, we understand deltas to take many forms, but the expectation of consistency is that deltas have bifurcations and spread their flow through multiple distributaries. This form, however, appears an artifact of the emphasis on fluvio-marine deltas. Closer examination of the many deltas that form in lakes reveal these terminal river deposits to have a consistently different form. Particularly, the bifurcations that define deltas in marine systems and give them that fanning delta shape are comparatively rare in lacustrine systems. Here deltas tend to from as elongate propagating channels transecting lakes. Even the minor distributaries of the fluvial-dominated bird foot delta are typically absent as lacustrine prograding channels.

Examination of ancient and modern lake deposits yield insights into the mechanisms which drive these core differences in delta morphology. Bifurcation is understood to be driven by the abundance of sand at the river mouth and the need for the flow to split about these blockages. Examination of prograding channels show sand is rare at the river mouth and the need for bifurcations are thus similarly rare. In all deltas the sand tends to lag behind the mud as the bed shear stress moving the sand is lost with the loss of slope but the mud is pushed on in suspension by flow momentum. Mud accumulating at the channel mouth builds subaqueous levees that emerge and become hardened by compaction and vegetation. Sand is evacuated during floods and typically overflows the levees of these elongated channels before reaching the channel mouth. The need for bifurcation thus never occurs. The resulting deposit is an elongated channel fill within lake mud that with thin sandy “blowout wings” off the channel edge. This scenario though common in lakes is rare in marine settings as the basin tide and wave processes common to marine and rare to lakes advect the mud and the muddy gun-barrel never forms and the sand reaches the mouth and the channel bifurcates. It is the marine basin processes that are responsible for familiar delta form.