AT 4,132 mi (6,650 km) long, the Nile River is the longest river in the world. It flows from two principal sources in equatorial Africa that join at Khartoum, Sudan, and continues north through Sudan and Egypt, emptying into the Mediterranean Sea. Historically, the Nile has been used in irrigation, farming, and transportation for thousands of years, from the beginning of Egyptian civilization into the 21st century. The Nile is fed by many tributaries, but ultimately it has two main sources: the White Nile in Burundi, and the Blue Nile in Ethiopia. Except when swollen by rains in August and September, the Blue Nile contributes less than 20 percent of the water; the White Nile provides the larger share. The ultimate source of the White Nile remained a tantalizing mystery until 1937, when Burkhart Waldecker discovered it while seeking asylum in the Belgian Congo from Nazi persecution in his native Germany. Waldecker pinpointed a stream that became the Kasumo River, flowing from Mount Kikizi in Burundi. This river eventually joins the Mukesenyi, the Ruvyironza, the Ruvubu and finally the Nyabarongo River. Also vying for recognition as the source is another small stream from Mount Bigugu that feeds into the Lukarara River and then joins the Nyabarongo.