deltas

deltas A delta constitutes both the alluvial tract of land at or near the river mouth (delta plain), the adjacent coastal zone (delta front) that is reworked and shaped by waves, and the offshore area below storm-wave base (prodelta) that is still influenced by the deposition of most fine-grained, river-derived material. Its morphology is the result of the river interacting with the receiving basin. Figure 1 shows how delta morphology is defined by fluvial processes (fluvial regime) on the one hand, and basinal processes (basinal regime) on the other. The fluvial regime is controlled by hinterland characteristics, such as drainage area, geology (e.g. lithological variation), and topographical relief of the catchment, which are all subject to continuous change. The rates at which these changes take place depend on the rate of variation in climate, tectonic, and sea-level change. For instance, periods with increased run-off at times of climate deterioration, or periods of tectonic uplift of the hinterland, will lead to an increase in sediment supply to the delta due to increased erosion rates in the hinterland. Other factors like vegetation and human modification also modify the input from the fluvial regime. The basinal regime is controlled by the geographical setting, shape, depth, and size of the receiving basin. These characteristics define its dynamics in terms of available tide and wave energy, but other variables, like the density of the basin water relative to the river water, also play a role in shaping the morphology of the delta.

Fluvial regime

The control of the fluvial regime on delta architecture is best shown by deltas prograding in low-energy basins, i.e. in basins where wave and tidal influence can be neglected. High gradient (>0.5°), closely spaced, and highly mobile distributary channels in the delta plain feed the delta front essentially as a line source (a more or less uniform supply of sediment along the delta front), which results in a typical, fan-shape delta-plain morphology (Alta delta, Fig. 2). Deltas built by these high-gradient streams are generally small in size and coarse grained, consisting predominantly of sand and gravel. A special example is the fan delta, a delta that is fed by a sandy to gravelly alluvial fan. In contrast, low-gradient (>0.5°), widely spaced, and relatively stable distributary channels in the delta plain feed the delta front essentially as a point source, which results in a typical lobate-shaped delta-plain morphology (birdfoot delta). The latter delta type is generally fine grained and develops at the coastal margins of extensive lowland areas. A good example is the Mississippi delta. The regular splitting and shifting of distributaries in the delta plain is caused by bars that are formed at the river mouth. There, the velocity of the river is checked by the standing water of the basin, causing deceleration of the river current, thereby reducing its transport capacity and promoting sedimentation. The deposited sediment causes an obstruction in the river mouth in the form of a bar which is coarse upstream and gradually fining downstream. As these mouthbars grow in size, they will deflect and split the flow, causing the river to bifurcate. This process will eventually lead to the characteristic lobate pattern of a fine-grained, fluvially dominated delta. Bar characteristics, such as width/length ratio, depend strongly on river-mouth processes, which are controlled by density contrasts between river and basin water, by the channel- to basin-depth ratio at the river mouth, and by the bed load to total load ratio. In the case of a line source, bars also develop but will coalesce to create a uniform delta front. The steepness of the delta front in low-energy basins depends mostly on the initial basin relief (e.g. absence or presence of a coastal shelf) at the river outlet, and the fluvial regime. Low-gradient delta fronts are often associated with rivers with a high suspension load to total load ratio. Examples include the many large, fine-grained deltas. Deltas with steep-gradient delta fronts (so-called Gilbert-type deltas named after G. K. Gilbert who, in 1885, described these deltas from Lake Bonneville) are often sandy to gravelly and associated with rivers with a low suspension load to total load ratio.

Basinal regime

The role of the basinal regime in creating delta morphology can be pictured as the relative contribution of wave and tide processes to river processes. Often a triangular diagram is used to distinguish between fluvial-, wave- and tide-dominated delta systems (front triangle of Fig. 2). Wave-dominated deltas tend to have straight delta fronts, because the wave energy that causes the sediment redistribution along the delta front is evenly distributed along the entire coast line. Examples include the Nile and the Rhône deltas which both have delta plains with a shape similar to the Greek capital letter D, with the apex of the delta pointing upstream. The characteristic ‘delta’ shape of wave-dominated deltas was first recognized by Herodotes from the Nile delta plain in the fifth century bc, and the term delta has since been used to denote all types of deltas. Tide-dominated deltas are characterized by tidal sand ridges that are oriented at a high angle to the coastline. Examples include the fine sandy Ganges/Brahmaputra delta and the Klang delta. The ridges develop best if the ratio of tidal to fluvial discharge is high. The importance of the type of distributary system is visualized in Fig. 2 by giving the ternary diagram a third dimension with the prevailing grain size.

G. Postma