Riverb, a sizable stream of freshwater flowing through a natural channel in the land. Streams too small to be termed rivers are called by such names as brook, branch, and creek. Most rivers flow on the surface, but in some areas they go underground for great distances.

To humans, rivers have always been important physical features. Along such rivers as the Nile, Tigris-Euphrates, and Indus some of the greatest of the early civilizations were developed. During medieval times, riverbanks provided the sites for cities and towns, and river valleys became great avenues of commerce.

Rivers are even more important in modern times. They supply water for home and industrial uses. They provide cheap water power and transportation. They give needed water through irrigation projects. They aid in the disposal of waste. Also, they are often of great scenic and recreational value. Along their banks are some of the world's greatest cities. In their valleys are some of the finest agricultural lands.

Rivers are among the most powerful natural forces in shaping the earth's surface. In draining the land of surplus water, rivers wear down mountains, plateaus, and other high landforms. In a never-ending process, eroded material is carried by rivers. Some is deposited to form floodplains in the valleys, some forms deltas at the rivers' mouths, and some is deposited in the sea. Given enough time and no disturbances of the land, rivers could wear down all heights to almost sea level.

Rivers flow in all directions, the only limiting factor being that they follow the slope of the land. Most rivers ultimately flow to the sea, but many rivers lead to landlocked lakes and seas or dry up in arid wastelands. The Volga River is the best example of one flowing to a landlocked sea—the Caspian Sea. Most rivers in the Great Basin of the western United States and in Xinjiang province of western China dry up in arid lands.