The largest river in the United States, the Mississippi River, has many tributaries. The Missouri River and the Ohio River are two large tributaries of the Mississippi. Rivers not only drain water from the areas around them, they pick up loose soil, sand and plants and carry them downstream. At the mouth of a river, where it empties into a sea or a lake, the loose particles can build up. The build up is called a delta.
Some deltas are very large.
The Nile River in eastern Africa is the longest river in the world and has a large delta. The Amazon River and Mississippi River also have large deltas. River deltas make good farmland, as does the land along rivers, called flood plain. This land is called flood plain because it is low and easily flooded when a river overflows its banks. Floodwaters carry soil and other particles and leave them on the flood plain.