A river is a body of water that flows. A small river is called a stream, a particularly large one is a river. The river begins at a spring, where water comes out of the ground. It ends at the mouth, where the river flows into the sea. Because rivers usually flow very shallow and sluggishly at the mouth into the sea, they deposit their sand there and form a triangular landform. This is called a delta. There, the river usually divides into several arms.
However, many rivers do not flow directly into the sea, but into another river. So they are only tributaries of the river that is the longest of them. Together, these rivers form a river system. It ensures that the water from a large area does not stay there, but flows away.
Rivers are therefore an important part of the water cycle: the sun heats the water of the sea. It evaporates into clouds, which also move over the land. There it rains. The rainwater becomes groundwater underground, and springs form in some places. The water from the springs then flows back into the sea via the rivers.
The longest rivers in the world are the Nile in Africa and the Amazon in South America. The Rhine is the longest river, flowing at least partly through both Switzerland and Germany. The Danube, which is much longer overall, flows through Germany and Austria. The Rhone flows through Switzerland and France. Even longer than the Rhine, Rhone and Danube is the Volga in Russia.