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Drainage basin (catchment area) is the area which supplies a river system, lake or reservoir with water. The whole area consists of smaller sub-catchments supplying tributaries of the main ri-ver and direct catchments, which drain straight into a lake or main river (Box 3.1).ts purpose is to co-nvey excess water from a drainage basin, which will include the products of weathering and addi-tional loads of solutes produced by man. This pro-perty makes a drainage basin an integrator and its operation is reflected in the quantity and quality of the river run-off.
Â­A watershed is an area of land that feeds all the water running under it and draining off of it into a body of water. It combines with other watersheds to form a network of rivers and streams that progressively drain into larger water areas. Topography determines where and how water flows.Ridge tops surrounding a body of water determine the boundary of a watershed. Imagine turning an open umbrella upside down in the rain.he umbrella is like a watershed; it collects everything that falls into it. Waterways within the watershed all feed into that main body of water, which could be a river, lake, or stream. The beginnings of a water source are called headwaters.