The territory of Ukraine is conditionally divided into 9 hydrographic zone according to major river basins including basins of rivers Wisla (Western Bug and San), Danube, Dniester, Southern Bug, Dnieper, Don, rivers of Black Sea littoral, Sea of Azov littoral, and separately rivers of Crimea. The biggest river basin by area is Dnieper which is subdivided into Prypiat basin, Desna basin, basin of Middle Dnieper, basin of Lower Dnieper. Beside Dnieper, basin of the Danube zoning is also subdivided into basin of Tysa, basin of Prut and Siret, and basin of the Lower Danube.