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Part 2

Understanding the system

This figure shows a 3D illustration of the landscape of any river, from its source in the mountains to its outflow into the sea. The text provides a summary of the natural dynamics in the main three parts of a river: the source in the mountains, the transition zone and the delta.

Natural dynamics

Rivers come in all sorts of shapes. Large rivers, generally, start to form high up in the mountains. Small streams join larger ones that, in turn, join other streams and gradually form a large, braiding river.

This river continually divides and converges around islands of sand and gravel. This sediment is eroded and deposited elsewhere, resulting in ever-changing patterns of channels and islands. When the river then leaves the mountains behind and the character of the landscape changes, so does the river.

More information on the impact of interventions and climate change on sediment transport and morphology:

The Rhine River case

Delta systems

As the interface between rivers and the sea, delta systems are highly dynamic systems, formed by the fresh water, sediments, nutrients and minerals supplied by the river, and the interaction with the sea, encompassing tidal and wave dynamics, inshore and longshore sediment flow, and sea level rise. Depending on the interaction of fluvial, tidal and wave processes, these deltas can take on all kinds of shapes. Most deltas are small, but some of them cover wide areas extending into several countries.

Deltas are highly productive systems and are often of high ecological importance as breeding, feeding and nursery areas for marine crustaceans, fish species and birds. Certain ecosystems, such as tidal forests and mangroves, depend on sediment influx and fresh water and saltwater dynamics.

River basins and their deltas stretch out from the smallest streams in the mountains to wide, estuarine outlets into the sea. There are thousands of river basins on our planet. Most of them are very small, but some cover wide areas that extend into multiple countries. At the outlets, rivers drop their sediments which, together with sediments coming in from the sea, often form deltas in coastal seas. Depending on the interaction of fluvial, tidal and wave processes, these deltas take on all kinds of shapes. Socio-economic developments, infrastructure and climate change strongly impact the functioning of rivers and deltas.

Select a delta type:

Wave dominated deltas

In almost 80% of the world’s river deltas, waves are the dominant force. These deltas have the deltaic shape that has given deltas their name. The sediment that is carried into the delta is subsequently pushed back again by waves, creating a smooth coastline.

Most of these wave-dominated river deltas are relatively small, however. The traditional deltaic shape may be the most common, but the discharge from the rivers of these deltas is only 16% of all river discharges into the seas.

±8700 wave dominated deltas worldwide. In total, these deltas make:

Number of deltas
Sediment flux

Source: Utrecht University, Nienhuis et al., 2020

Wave dominated deltas

This figure shows three types of deltas
Those that are mainly formed by waves in coastal zones, those formed by tides, and those formed by river discharge and sediment outflow into the sea. By clicking on a delta type in one of the three circles on the right, the occurrence of this delta type is shown on the world map below. Wave-dominated deltas are the most common. They are relatively small, compared to the other types, since those have much larger sediment input from rivers.