The fine bits of rock and soil deposited on land by a river are called alluvium.

Here's a breakdown:

* Alluvium is the collective term for sediment transported and deposited by a river.

* This sediment can range in size from clay (very fine) to gravel (larger pieces), and includes everything in between.

* River deposits are a key part of the process of land formation and change. They create fertile soils and contribute to the development of river valleys, floodplains, and deltas.

So, when you see a rich, fertile area near a river, it's likely thanks to the accumulation of alluvium!