A drainage basin is an extent of land where water from rain and melting snow or ice drains downhill into a body of water, such as a river, lake, reservoir, estuary, wetland, sea or ocean.
The drainage basin includes both the streams and rivers that convey the water as well as the land surfaces from which water drains into those channels, and is separated from adjacent basins by a drainage divide.
Each drainage basin is separated topographically from adjacent basins by a geographical barrier such as a ridge, hill or mountain, which is known as a water divide.
The drainage basins in South Africa do not correspond with the Water Management Areas, and have the letters A, B, C, D, E, F, G, H, J, K, L, M, N, P, Q, R, S, T, U, V, W, and X.
What seems to be the case though is that each area refers to some major river systems and their tributaries (a region for each major river system).