Formation of river deltas

Schematic formation of deltaic distributaries and inter-levee basins. From Axelsson 1955 (Ymer 75).

At the mouths of the distributaries where the flanking levees are low and poorly developed, the river water spreads laterally and mixes more or less with the water of the adjoining basin. The marginal diffusion and deceleration of the expanding flow results in a deposition of bed load and suspended load that often creates a broad concentric distributary-mouth bar, (A1, A2). As the bars grow in size they will deflect and split the flow and often cause the channels to bifurcate (B1, B2, B3, B4). As the new channels extend basinward they may bifurcate again and sometimes channels that converge upon each other may merge. If the splitting, extension and merging or re-joining of channels is followed by the development of new, more or less divergent and convergent levee systems, inter-levee basins (C1, C2, C3) may be embayed and later cut off from the frontal lake or sea basin. During high discharges minor overflow channels may be formed, widening and deepening at the outside of bends, and especially at the distal part of the channels where the levees are low and poorly developed and the vegetation is sparse. The majority of these channels are short-lived, some may evolve to larger distributaries while some may come to function as smaller channel connections (D1, D2) with the inter-levee basins. The passage at the distal part of embayed areas often remains open for many years after it has been cut off from direct connection with the frontal lake or sea basin. Sometimes this passage (E) develops into a channel that carries turbid river water into the inter-levee basin during rising river stages and clearer basin water out to the river channel during falling river stages. If the inter-levee basin has a deeper channel connection further upstream, the distal channel connection will mainly function as an outlet. The basin may then be subdivided by levees formed along the course of the through flow.

The development of a branching channel pattern is dependent on the processes taking place at the head of channel forks. Often there is a tendency to a concentration of the flow to the channel, that is favoured by the greatest slope (energy gradient) This channel therefore tends to be larger and larger at the expense of the smaller and less efficient ones. Due to aggradation in the main channel, part of the flow may be diverted to an existing side channel or to a new crevasse channel, formed at high flows through breaks in the levees and favoured by a greater slope. The helicoidal flow conditions (spiral flow) at a channel bend favours erosion on the outside and deposition on the inside and a slow downstream migration of the (meander) bend.

The closing of one branch at a channel fork usually demands that the other branch is or may become capable, by erosion, of holding a larger flow. Since this is often the case, channel closing may take place when the equilibrium conditions are changing, and the competency and capacity of one branch decreases or increases in relation to the other.

Channel splitting will therefore often be followed by channel closing and by a concentration of flow within the older parts of a fine-grained delta to fewer, active distributaries. During the constructional phase these tend to become larger and larger at the expense of the smaller and less efficient ones.

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