River, long channel of water. The ground beneath is called the bed; to either side are its banks. Rivers begin as headwaters overflowing from lakes or running down mountains as the snow melts, forming rills, brooks, and streams. The amount of river water depends on rainfall, since the river system provides the drainage for the surrounding land. The water runs downward to sea level, taking the shortest, steepest route; the river's upper course has the swiftest currents, as well as any waterfalls or rapids. The force of the current may erode the valleys or cut into rock, forming canyons. The river's lower course usually flows through a flat area called the flood plain until it reaches the mouth, the point where the river reaches the coast. The mouth may form a delta (a triangular deposit of sediment), or an estuary (a deep, wide mouth filled with fresh and salt waters). Rivers, important routes of transportation, can provide power for industry and help irrigate crops. At 4,145 mi (6,671 km), the Nile River in Africa is the longest in the world.