How does the Danube River form part of the border between Romania and Ukraine?

The Danube River forms part of the border between Romania and Ukraine through its lower course as it approaches the Black Sea.1

Near Tulcea, the river begins to spread into its delta and splits into three primary channels. The northernmost of these, the Chilia Channel, serves as the boundary between the two nations. This channel is the most significant of the three, carrying approximately 63 percent of the river's total runoff.12