Rila is an important hydrological unit in Bulgaria with very high hydropower potential, representing about 1/4 of the total potential of the country. The water reserves that form within the mountain range are the most important source of clean potable water for the surrounding settlements, the national capital Sofia and part of the population of Northern Greece and European Turkey. The regime of the rivers of Rila is directly related to the altitude and about half of the water reserves in the mountain are at an altitude above 2,050 m. The main drainage divide in the Balkans separating the drainage basins of the Black Sea and the Aegean Sea passes through Rila's northern ridge, including the summit of Musala. Some of the Balkans' longest rivers originate from Rila, including the Maritsa, Iskar and Mesta, as well as several important tributaries of the Struma — Rilska, Dzherman and Blagoevgradska Bistritsa. Of them, the Iskar and its tributaries belong to the Black Sea drainage basins, and all the rest — to the Aegean. About 78% of the water flows into the Aegean drainage system. The runoff comes from snowmelt in the Alpine zones and rainfall. Water discharge reaches its maximum in late spring and early summer with spring accounting for more than half of the total annual discharge. The maximum at the highest altitudes is in summer due to the late snowmelt. The minimum is in autumn and winter. The largest waterfalls are Skakavitsa (70 m) and Goritsa (39 m). There are abundant mineral springs that include the hottest one in South-eastern Europe at Sapareva Banya forming a geyser with a temperature of 101.4 °C.