By rivers, South Wales

Most rivers, as they near the sea, start to meander across a floodplain. The river is still eroding its banks along the outsides of meanders, where the flow is fastest. But on the insides of bends, where flow is slow, it deposits sand and gravel, so that the meanders become larger. They may even meet together, cutting off a meander to leave an oxbow lake.

When the river floods, it flows out across the floodplain and slows down, depositing finer silt and mud. This forms fertile farmland, but damages property built in unwise places!