The entire geographical area drained by a river and its tributaries. It is defined by topographic ridges that form water divides separating adjacent basins.
A drainage basin is the total land area from which water drains to a common point along a stream or river system. Its boundaries, called drainage divides or water divides, follow topographic ridgelines where precipitation falling on one side flows toward one outlet and precipitation on the other side flows toward a different outlet. Drainage basins are hierarchically organized, with smaller sub-basins nesting within larger basins. Key morphometric parameters of a drainage basin include area, perimeter, relief, drainage density, stream order, and shape factor, all of which influence the hydrologic response to rainfall. The concept of the drainage basin is fundamental to hydrology because it provides a natural unit for water balance calculations, where inputs (precipitation) equal outputs (streamflow, evapotranspiration) plus or minus changes in storage. Major world drainage basins, such as the Amazon, Congo, and Mississippi, shape regional climate, ecology, and human settlement patterns. Drainage basin management integrates land use planning, water quality protection, flood control, and ecosystem services assessment.