A river starts on high ground, usually in hills or mountains, where water collects and flows together to form streams. When many streams come together, they are called a river. The place where the river starts is called its source.
The first small streams that are formed as water collects and flows downhill are called gullies. Gullies come together to create larger streams. Streams continue flowing downhill, meeting at places called confluences. When two streams meet, the smaller stream is referred to as the tributary of the other stream. Eventually enough streams come together to form a river. The river continues to flow downhill until it flows into a lake, sea or ocean at its mouth, according to About.com.