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Deltas: Young, Fragile and Threatened Environments

Deltas are structures formed from sediments of mainly fluvial origin, built by a set of processes that combine the actions of rivers and the sea; they exhibit a set of fluvial and coastal forms and, in general, a morphology that protrudes from a coastline that may be more or less exposed.

Effectively, delta formations are constructed when sedimentary materials of continental origin hold more sway than the destructive action of the receiving aquatic environment; the latter being due, either independently or jointly, to long-shore drift, swell, waves and tides.

Delta formations have developed over the last 6,000 years1, approximately, in other words since the relative stabilization of the Earth's sea levels, at the mouth of watercourses entering an ocean or a sea.

Fluvial sediments are deposited when water flow coming from the continent, having lost its power when entering the marine environment, is forced to deposit its load. Deltas are developed fully when they extend large fluvial bodies that drain vast watersheds subject to high levels of erosion and that carry abundant sedimentary flows. Conversely, a river with little input of sediment ends with an estuary that is more or less filled in and subject to tidal effects.

1.1. Long-term construction of deltas: general mechanisms

1.1.1. Processes and basic forms

Delta formations generally fall into one of three types of basic formations and sedimentary facies. Regarding the sea level, deltas are organized from upstream to downstream in the following simplified way.

1.1.1.1. Delta plains or subaerial deltas

The upper delta plain (channels not affected by salt water) and the lower delta plain (subject to tidal effects) can be distinguished by the limit of influence of the tidal regime; this limit depends on the slope of the delta and the flow rate in the watercourse. The distribution channels* are produced by avulsion*, which creates a new channel when the main channel rises (or vertically aggrades) by sediment deposition, and a flood opens a breach in one of the levees bordering the channel. The levees, made up of sandy materials, are separated by shallow basins that form freshwater or briny wet zones which are rich in ecology. The general facies of the deposits features beds inclined at low angles at the surface; aggradation of the delta plain is slow because materials are transported at the surface towards the delta front.

1.1.1.2. The delta front or proximal part

The front combines a narrow subaqueous platform and the "front" itself, which progresses (or "progrades") more or less rapidly depending on the intensity of fluvial input, generally sandy and/or silty. The front has a topographic slope of 10° to 25°, which conforms to the dip of deposits brought down to the mouth.

1.1.1.3. The prodelta or distal part

The prodelta is the subaqueous part of a delta that rests on the continental shelf*. The prodelta, created in a zone of deep water with an ocean floor of gentle slope, is made up of very fine deposits, silts and clays from suspended loads carried by plumes. The deposits are made up of laminated beds. This unit is itself fossilized by the progressing delta front.

1.1.2. Dynamics of construction and redistribution in progress

The plan form of a delta is conditioned by the interrelation of three competing forces specific to the receiving environment, one fluvial and the two others marine in nature; each of these forces can win over the others depending on the intensity level of its action. This set of influences is the basis for ternary classification of deltas, on a genetic basis, that remains the most widely used [BHA 06, GAL 75].

1.1.2.1. River-dominated deltas

River-dominated deltas are subject to the dominant combined action of the liquid and solid flow rates of the river (made up of the bed load* and the suspended material*). These deltas present characteristics of formations undergoing active construction, in contrast to the two following types. They have an elongated shape, whether it is simple or composite, constructed as an extended prolongation of the lower delta plain. The distribution channels or branches end in sandy mouth bars*. The branches separate bays that are progressively transformed into marshes. The Mississippi Delta is the archetype of a river-dominated delta, at least in its northern part, which is constructed along the main fluvial channel (Figure 1.1).

Figure 1.1. Typology of deltas as a function of greater or lesser influence of the river, tides and waves. (Source: [FIS 69] and [GAL 75], redrawn by F. Salomon). For a color version of this figure, see www.iste.co.uk/bravard/sedimentary2.zip

1.1.2.2. Tide-dominated deltas

These deltas are formed from the branches of distribution channels shaped in a lower delta plain by the action of tides; the branches are called "estuaries*"; the average tidal range is felt upstream and its influence is dominant in comparison with the influence of the river. Delta branches under tidal influence (flood* and ebb*) are the most stable. Delta construction extends slightly or not at all into the sea, apart from with subaqueous mouth bars lying in an extension of the distribution channels and shaped by tidal effects. Between the mouths, vast areas of mud can develop on the tidal flats*, often intersected by thin sandy bars called "cheniers*", and colonized by mangroves which are forested formations adapted to salt water. This type of delta is well illustrated by the Amazon, Ganges and Brahmaputra Deltas, or even by the Yangtze Delta.

1.1.2.3. Wave-dominated deltas

The coarsest fraction of sediments carried down by the river (in general, sands that belong in part to the bed load and in part to the suspended load) is redistributed more or less actively by the waves (mainly as a function of their average height) and by long-shore drift*, in sandy bars parallel to the coastline of the delta; these bars or barrier beaches have seasonal openings and may isolate lagoons. Under the action of waves, the deltaic protuberance takes on a lobed form, often pointed in shape. When the deltaic lobe* loses input from fluvial materials, it erodes, regresses and melts into the coastal bars, shaping the shoreline between the active mouths. This type of delta is seen in the Senegal and Nile Deltas, as well as in the Tiber and the Rhône in the Mediterranean.

While this classification is convenient, in practice deltas often take on mixed and evolving forms, in particular by the association of forms reworked by waves and tides, whether the delta is progressing quickly or slowly under the influence of materials of continental origin. This is why certain specialists look at the balance between the suspended load of rivers at their mouth on the one hand, and the relative importance of the action of waves and tides by means of their ratio on the other; this is a means of providing a qualitative basis for grouping deltas together in large families [HOR 07a]. The deltas of the Orinoco, the Red River and the Mekong belong to a mixed type. The Mekong Delta has sandy coastal bars shaped by waves, as well as estuary-type branches in its northern part and mangroves to the south, where muds are deposited downstream of the direction of long-shore drift.

Furthermore, the proportion of sand and mud coming from the continent conditions the formation and future of deltas. Deltas with a high proportion of sand (more than 10% of the total load) have a greater area above sea level than mud deltas; however, the presence of mud is essential because it consolidates soils, makes them more resistant to fluvial and marine erosion and encourages good health in vegetation formations such as marshes and mangroves. The latter two increase the resilience of the delta to the forces applied to it by the ocean [GIO 14].

1.1.3. Young and unstable areas

1.1.3.1. Natural sinking and delta equilibrium

Natural or geological subsidence is a slow and regular movement that reduces the level of the ground surface. Several processes can exist, sometimes simultaneously:

• - on the one hand, a deep process can sometimes be at play, generally related to the movement of the lithosphere, for example linked to downward movement due to fault tectonics or to large-amplitude warping that affects sub-delta sedimentary deposits;
• - on the other hand, at a different timescale (of climate periods within the Quaternary epoch) and in the regions of the Earth that have undergone glaciation, these vertical movements can also be isostatic* in nature, and will become more obvious as they approach the poles. Under the weight of ice, portions of continent have sunk by several meters, causing an equilibrium process with uplift of their external marginal areas; this is called an "isostatic adjustment". During melting of ice sheets, the process is inversed: deglaciated areas are uplifted and their marginal zones sink; this is called "isostatic rebound";
• - finally, subsidence can be due to compaction of sediments that have been deposited in delta formations. The recent age of deposits (see the later section) means that they have not yet...