Do rivers only flow south?

Water flows downhill, and downhill has no relationship to north or south. The belief that rivers flow south comes from the convention of drawing maps with north at the top, which makes downward look like a natural direction of travel and southward feel like descent. The Nile flows north through Africa for more than 4,000 miles from the highlands of East Africa to the Mediterranean. Russia's Ob, Yenisei, and Lena all flow north into the Arctic. The Rhine flows north to the North Sea. Florida's St. Johns River flows north as well, dropping only about 30 feet across its entire length.

North-flowing rivers create a specific hazard worth mentioning. In the Siberian and Canadian Arctic, the upstream headwaters thaw in spring before the downstream mouth does, so meltwater arrives at a channel still blocked by ice and backs up, producing extensive seasonal flooding known as ice-jam flooding. The map convention itself is also arbitrary rather than natural: medieval European maps often placed east at the top toward Jerusalem, and Egyptian and some Islamic maps put south at the top. North-up became standard largely through European navigational practice and the magnetic compass, not because it reflects any physical property of the planet.