Think of the veins and arteries in your body. They carry the blood that nourishes you and keeps you going. You can think of rivers in the same way.
Rivers are the veins of the earth, carrying the water that all life needs to survive.
A river's headwaters could start as snow melting from rocky mountain peaks or as a little stream bubbling up from a forest floor. These trickles, also called rills, form streams. When enough streams flow together, they form a river. The streams that flow into rivers are called the river's tributaries. As a river continues downstream, it typically runs more slowly over leveler ground.