A river is a natural channel down which water flows to the sea or a LAKE. Throughout history, rivers have provided water for drinking, farming, and industry, and offered food, transportation, and recreation. Some of the world’s largest cities have grown up on river banks.
Rivers usually begin as a trickle of water high in hills or mountains. Some come from rainwater or melting snow. Most emerge from underground streams, formed after rain or snow seeps into the ground then bubbles back to the surface. As the water flows downhill, the trickle swells into a stream and then, as side streams called tributaries join it, into a wider river.
As a river flows, it carries along material, or debris, called its load. A river’s load includes rocks, stones, and other large particles, which are washed along the riverbed. Finer particles float in the water.
A river’s load scours the riverbed, deepening its channel. The speed of the flowing water erodes the river’s banks, making it wider. As the river winds through the landscape, it gradually carves out deep valleys in solid rock and deposits huge amounts of debris to form a fertile plain. In places where the river flows over soft limestone, water seeps into the rock, slowly dissolving it and forming tunnels and caves.
As a river flows into the sea, it often widens and forms a broad inlet called an estuary. The sea’s tides carry salty seawater upriver to mix with the fresh river water. The salt makes tiny particles of clay in the fresh water clump together and sink, often causing sediment to build up at the river’s mouth.
A delta is an area of flat, fertile land at a river mouth. Deltas form when a slow-moving river deposits its load of sediment as it reaches the ocean. The sediment slowly builds up and dries out, forcing the river to split into separate channels.
A lake is an expanse of water that forms inland where water collects in a hollow in the ground and cannot drain away through the rock below. Most lakes are fed by rivers and, to a lesser extent, rainfall.
Small lakes called tarns form in mountains where glaciers gouge out bowl-shaped hollows. Water collects in the craters of inactive volcanoes to form volcanic lakes. The world’s largest lake, the Caspian Sea, lies in a hollow created by geological upheaval. Artificial lakes called reservoirs are created by dams.