Where a river puts down its load
Why Do Rivers Fan Out at the Sea?
A river carries dirt for a thousand kilometres and drops all of it in the last few, building new land as it goes.
-
Step 1 of 5
Moving water carries; still water drops.
Fast water holds grains of rock in suspension. The moment it slows, it cannot hold them any longer and everything it was carrying settles out.
-
Step 2 of 5
The sea is where a river stops.
A river meeting the sea has nowhere left to run downhill. It spreads out, slows almost to nothing, and unloads in the space of a few kilometres.
-
Step 3 of 5
The pile blocks the river's own way.
Dropped sediment builds up until the channel is choked. The river bursts out sideways, finds a new way through, and starts choking that one too.
-
Step 4 of 5
Repeat for centuries and you get a fan.
Every abandoned channel leaves a ridge of its own. Over a long time the whole mouth becomes a spreading fan of land shot through with branching water.
-
Step 5 of 5
Delta soil is extraordinary farmland.
Everything dropped there was ground off a whole river basin, so it is fine, deep and full of minerals. Some of the most crowded farmland on Earth sits on deltas.
Delta land only stays above water while the river keeps feeding it. Dam the river upstream and the delta starts to sink and shrink.
The short version
A river drops its load the moment it slows, chokes its own channel with the pile, and bursts sideways to start again — centuries of that is the fan. That soil, ground off a whole basin, is some of Earth's best farmland.
Try it yourself
Stir soil into a jar of water and let it stand. The order things settle in is the order a delta builds.