A drainage basin is an area that drains all of its water towards a common outlet, a watercourse or the sea.
There are different types of drainage basins depending on the scale of work or study, from plots of a few dozen square meters of a tiny tributary up to the continental fronts that feed the oceans.
A drainage basin is limited by natural borders: ridges or continental divides. On either side of these borders, water from precipitation and springs, as well as all dissolved or suspended elements (e.g. sediment, pollution), flow towards separate outlets.
The drainage basin is the right place to address causes upstream of surface water issues: water shortage, pollution, fish migration, etc.
In purple, the limits of the Authie drainage basin. © Antonov14, Wikimédia CC by-sa 3.0