Ukraine has around 23,000 rivers. Most of the rivers of Ukraine drain into the Black Sea and Azov Sea belonging to the bigger Mediterranean basin. Those rivers mostly flow in southern direction, except for some Pripyat tributaries in Volhynia and Dniester tributaries in Prykarpattia. A small portion of rivers is part of Western Bug drainage basin located in Western Ukraine near the border with Poland. Those rivers usually have northern direction and are part of bigger Baltic basin. The most notable rivers of Ukraine include: Dnieper, Dniester, Southern Buh, and Siversky Donets. The longest river is Dnieper, the longest tributary is the Dnieper's tributary Desna. Two of the Danube's tributaries in Ukraine Prut and Tysa are longer than the main river within Ukraine.