A river is a naturally-winding channel carrying water through the landscape. The sides of a river are called the river banks. They are usually made of fine materials. The top of the riverbank is made of soil.
Small rivers are called brooks and streams. There is no exact definition of when a stream becomes a river. However, usually brooks and streams are small enough to be jumped across, or they can be waded. Rivers are generally too big to jump across, or too deep to be waded.
The river is fed by water seeping out of the banks. Water sinks into the soil as rain, and then drains slowly through the soil (and also sometimes through rock) until it reaches the riverbank. Soil and some rocks are natural sponges. They hold on to water and also only allow the surplus to flow out slowly. This slow flow explains why rivers do not dry up when it is not raining.
However, if rain falls for too long, the soil will fill up and then the rest of the rain will flow over the surface and reach the river much more quickly. Also, if it rains so hard the soil cannot soak it all in fast enough, the rest will flow over the soil surface. When water flows over the surface, it reaches rivers very quickly. Then the river cannot carry it away fast enough and the rest spills out over the banks, causing flooding.
The area that feeds water into a river is called 'the drainage basin' or the 'river basin'.