The wide flat area of land beside a river is called a floodplain, for obvious reasons.
When it rains, water feeds into the river and the river level rises.
When the channel is completely full of water, it is known as bankfull. At that point, if any more water is added, the river floods.
The floodplain is so flat that as soon as the river overflows on to it, the water can easily spread out over a large area.
Yet again, we can interfere and engineer the landscape to prevent flooding, but the river also creates its own flood prevention barrier.
You see, when the river has filled its channel to bankfull it is usually pretty deep and flowing fast. This enables it to carry lots of stones and mud.
However as soon as water tips over onto the floodplain, that bit is suddenly just a shallow layer. It is not being pushed along in any particular direction and the movement is much reduced.
Slower flow means less energy to carry stones and mud, so as the river overflows its banks and slows down it dumps stones and mud right there beside the channel. This raising of a river bank is called a levee.
Now don’t expect to see a huge heap of stuff. However, if you stand on the river bank and walk away onto the floodplain, you often begin by walking slightly downhill.
It’s incredibly hard to show in a photograph, so you will just have to believe me or go out to a big river with a flat floodplain and take a look for yourself.
And where you do see a huge heap of stuff, that is usually due to another spot of river engineering, adding to natural levees, to reduce the risk of flooding.