A delta is land the river is still delivering, and the delivery has slowed

A delta is a sediment surplus with a shape

A river carries solid material as well as water, and when it reaches a standing body of water it slows and can no longer keep that material in suspension. The sediment drops, the channel silts up, and the river eventually breaks out sideways and starts building somewhere else, which is how a delta grows.

Whether a delta forms at all depends on a balance. If waves and currents remove sediment faster than the river delivers it, no delta develops and the river simply enters the sea through a funnel-shaped estuary instead. Deltas are therefore commonest where the receiving water is relatively calm and the sediment load is high.

The shape records which force won. A fan of distributaries spreading outwards indicates a river-dominated system; a smooth arc suggests waves have been redistributing the sediment along the coast; a set of long fingers usually means tides are pushing and pulling the material back and forth in the channels.

The channel moves, and the settlements have to follow

A delta river doesn’t keep one route to the sea. Over decades and centuries it switches between distributaries as each one silts up and loses its gradient, abandoning the old channel and building a new lobe of land elsewhere. Ports founded on a working channel have been left inland by this process more than once.

That instinct to switch is the central engineering problem of every inhabited delta. Keeping a river in the channel that the ports, the cities and the irrigation systems were built around means embanking it, which prevents the flooding that used to spread sediment across the surrounding land.

So the sediment goes somewhere else. Confined between levees, the river carries its load past the delta plain and dumps it offshore, and the land surface that used to be topped up annually stops being topped up, which turns out to matter more than it sounds.

Deltas subside, and that is normal until the topping-up stops

Freshly deposited sediment is waterlogged and loosely packed, and it compacts under its own weight for a long time afterwards. A delta surface is therefore always sinking, and in an unmodified system that sinking is offset by fresh material arriving with each flood.

Remove the floods and the compaction continues alone. Add the extraction of groundwater or of oil and gas, which removes fluid from the pore spaces below, and the rate of subsidence can increase considerably. The result is a land surface dropping relative to the sea for reasons that are largely local.

This is why the vulnerability of deltas is a different problem from sea level in general. In many of the world’s large deltas the ground is going down faster than the water is coming up, and the two effects combine, which is a well-documented concern rather than a speculative one.

Dams upstream change the coast hundreds of kilometres away

A reservoir is a settling tank. Water slows behind the dam, sediment drops out and stays there, and what continues downstream is comparatively clear water carrying much less material than it did. That is a useful outcome for the reservoir operator and a consequential one for the delta.

Deltas fed by heavily dammed rivers have measurably less material arriving to replace what compaction and waves remove, and several are now eroding rather than building. The change is not caused by anything happening at the coast, which makes it difficult to address at the coast.

Sand extraction from riverbeds has a similar effect for a similar reason. Material removed from the channel for construction is material that never reaches the mouth, and where extraction is unregulated it can rival dams as a cause of sediment shortfall.

What a delta is like to travel in

A delta is a landscape organised around water rather than around roads, and travel across one is often slower than the map suggests because the direct line crosses a channel with no bridge. Local transport tends to be by boat, and the ferry crossings are the real junctions of the network.

The compensations are considerable. Delta plains carry enormous quantities of birds because they combine shallow water, mud and productivity, the fishing and farming are intensive and visible, and the settlements sit on whatever ground is marginally higher, which makes the topography easy to read once you know to look for it.

It’s also a landscape where the human works are the scenery. Embankments, sluices, pumping stations, drainage ditches and reclaimed polders are the dominant features of most inhabited deltas, and understanding them as a single water-management system makes them far more interesting than they first appear.