Origins of river water

A river may have its source in a spring, lake, from damp, boggy places where the mud is waterlogged, from glacial meltwater, or simply from rain flow off impermeable rock or man-made surfaces. Almost all rivers are coupled by other rivers and streams termed tributaries the highest of which are known as headwaters. Water may also be recruited to a river from ground-water sources. Right through the course of the river, the total volume transported downstream will often be a combination of the free water flow together with a substantial part flowing through sub-surface rocks and gravels that underly the river and its floodplain. For many rivers in large valleys, this unseen part of flow may greatly exceed the visible flow.From their source, all rivers flow downhill, naturally terminating in the sea or in a lake. In arid areas rivers sometimes end by losing water to disappearance. River flow may also be lost by percolation into dry, porous substance such as sand, soil, or into pervious rock.
Excessive abstraction of water for use in industry, irrigation etc can also cause a river to dry before getting a lake or the sea.The mouth, or lower end, of a river is recognized by hydrologists as its base level.The area drained by a river and its tributaries is called catchment, catchment basin, drainage basin or watershed. The term "watershed" is also used to mean a boundary between catchments, which is also called a water divide.