Portions of a geographical area which constitutes a hydrogeological unit as the catchment area for a single river which are under the jurisdiction of two or more countries. (Source: GEMET/WPRa)

Transboundary waters means any surface or ground waters which mark, cross or are located on boundaries between two or more States; wherever transboundary waters flow directly into the sea, these transboundary waters end at a straight line across their respective mouths between points on the low-water line of their banks (Source: 1992 Water Convention, Art.1)