Romania's lowest land is found on the northern edge of the Dobruja region in the Danube Delta. The delta is a triangular swampy area of marshes, floating reed islands, and sandbanks, where the Danube ends its trek of almost 3,000 kilometers and divides into three frayed branches before emptying into the Black Sea. The Danube Delta provides a large part of the country's fish production, and its reeds are used to manufacture cellulose. The region also serves as a nature preserve for rare species of plant and animal life including migratory birds.

After entering the country in the southwest at Bazias, the Danube travels some 1,000 kilometers through or along Romanian territory, forming the southern frontier with Yugoslavia and Bulgaria. Virtually all of the country's rivers are tributaries of the Danube, either directly or indirectly, and by the time the Danube's course ends in the Black Sea, they account for nearly 40 percent of the total discharge. The most important of these rivers are the Mures, the Olt, the Prut, the Siret, the Ialomita, the Somes, and the Arge . Romania's rivers primarily flow east, west, and south from the central crown of the Carpathians. They are fed by rainfall and melting snow, which causes considerable fluctuation in discharge and occasionally catastrophic flooding. In the east, river waters are collected by the Siret and the Prut. In the south, the rivers flow directly into the Danube, and in the west, waters are collected by the Tisza on Hungarian territory.

The Danube is by far Romania's most important river, not only for transportation, but also for the production of hydroelectric power. One of Europe's largest hydroelectric stations is located at the Iron Gate, where the Danube surges through the Carpathian gorges. The Danube is an important water route for domestic shipping, as well as international trade. It is navigable for river vessels along its entire Romanian course and for seagoing ships as far as the port of Braila. An obvious problem with the use of the Danube for inland transportation is its remoteness from most of the major industrial centers. Moreover, marshy banks and perennial flooding impede navigation in some areas.