Formation of the Balkan landmasses involved a number of earth crust foldings and volcanic actions that either dammed rivers or forced them into new courses. The flat basins that occur throughout the country were created when river waters receded from the temporary lakes that existed while the rivers were cutting their new channels. The largest of these is the Sofia Basin, which includes the city and the area about fifteen miles wide and sixty miles long to its northwest and southeast. Other valleys between the Stara Planina and the Sredna Gora ranges contain a series of smaller basins, and similar ones occur at intervals in the valleys of a number of the larger rivers.
From a drainage standpoint, the country is divided into two nearly equal parts. The slightly larger one drains to the Black Sea, the other to the Aegean. The northern watershed of the Stara Planina, all of the Danubian plateau, and the thirty to fifty miles inland from the coastline drain to the Black Sea. The Thracian Plain and most of the higher lands of the south and southwest drain to the Aegean Sea. Although only the Danube is navigable, many of the other rivers and streams have a high potential for the production of hydroelectric power and are sources of irrigation water. Many are already being exploited.
Insignificant when compared with the watersheds that drain to the seas, about 125 square miles of the country drain into a few small salt lakes that have no outflowing water. The largest such lake has a surface area of 2.5 square miles.
By far the greater part of the country that drains to the Black Sea does so through the Danube. Most of its major tributaries in the country (from west to east, the Ogosta, Iskur, Vit, Osum, Yantra, and Lom) carry more water than do the combination of the Provadiyska, Kamchiya, Fakiyska, and Veleka rivers, all of which flow directly into the Black Sea. Of the Danube's Bulgarian tributaries, all but the Iskur rise in the Stara Planina. The Iskur rises in the Rila and flows northward through a narrow basin. Territory not far from the river on both sides of it drains in the opposite direction, to the south. The Iskur passes through Sofia's eastern suburbs and cuts a valley through the Stara Planina on its way to join the Danube.