A surface basin is a topographically defined area that is drained by a water stream or common stream system in such a way that its drainage is discharged by a single point such as its outlet, a measuring or special interest section.The contour of a basin is delineated by a water dividing line, which separates the drainage of each river system.The water divide follows a rigid line, crossing the main course only at the starting point; or at the point of interest.