Rain and other water on the earth’s surface are constantly being carried off. Rivers are the larger streams that accomplish this task. Streams smaller than rivers are brooks. And still smaller streams are rivulets. These flow together and join until the growing stream may become a large river.
Many rivers flow into the sea. But some rivers flow into inland lakes, and rivers that enter dry plains may even grow smaller and smaller until they disappear by evaporation or by sinking into the dry soil.
River water comes in part from rainwater that flows along the ground into the stream channel. Or the river water may come from melting snow and ice, from springs, and from lakes.
Large rivers have many tributaries, or smaller streams, that flow into the main stream. The Ohio and Missouri—which are giant rivers themselves—are really tributaries of the still greater MissiSSippi. Each tributary has its own smaller tributaries, so that a great river system like the Mississippi is composed of thousands of rivers, creeks, brooks, and rivulets.
The land drained by a river system is called its “drainage basic”, or “watershed”. The Missouri-MiSsissippi, which is about 3,890 miles long, drains about 1,243,700 square miles. The Amazon River, some 3,900 miles long, has a watershed of over 2,722,000 square miles.
Rivers wear away the land and carry it, bit by bit, into the sea. For thousands of years, this can cause great erosion of the land. The Grand Canyon is the world’s best example of how rivers can cut great valleys into the land.