Almost all of Serbia's rivers drain to the Black Sea, by way of the Danube river. The Danube, the second largest European river, passes through Serbia with 588 kilometres (21% of its overall length) and represents the major source of fresh water. It is joined by its biggest tributaries, the Great Morava (longest river entirely in Serbia with  of length), Sava and Tisza rivers. One notable exception is the Pčinja which flows into the Aegean. Drina river forms the natural border between Bosnia and Herzegovina and Serbia, and represents the main kayaking and rafting attraction in both countries.