 After entering the country in the southwest at Bazias, the Danube travels some 1,075 kilometers (almost 40% of its entire length) through or along Romanian territory, forming the southern frontier with Serbia 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 Mureș River, the Olt River, the Prut, the Siret River, the Ialomița River, the Someș River, and the Argeș River.