The river whose delta is shared mainly by Romania and Ukraine is the Danube.
The Danube Delta forms where Europe’s second-longest river empties into the Black Sea. Most of the delta lies in Romania’s Tulcea County, while its northern section, especially along the Chilia arm, lies in Ukraine’s Odesa Oblast. The delta’s approximate area is 4,152 square kilometers, excluding some related southern lagoons.
Three principal distributaries carry the river toward the sea: the Chilia, Sulina and Sfântu Gheorghe branches. The Chilia is the longest and forms much of the Romanian-Ukrainian border zone; Sulina is the central navigable branch, and Sfântu Gheorghe is the southern branch.
The delta is internationally important for wetlands and wildlife. The Romanian portion became a UNESCO World Heritage Site in 1991, and the wider Danube biosphere reserve shared by Romania and Ukraine was established in 1998. Prut, Dniester and Dnieper are nearby or regionally important rivers, but none forms this cross-border delta.