an elevation 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 elevations 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. The cirques at the high elevation zone contain 189 glacial lakes; there are also about 30 smaller ones that evaporate in summer, including tectonic lakes like Panichishte. Their location is closely linked to the snowline during the last glacial period and most of them lie at elevations between 2,100 m and 2,500 m. Most lakes (28) are situated at elevations between 2,300 m and 2,350 m; there are 23 between 2,350 m and 2,400 m, 19 between 2,250 m and 2,300 m and 19 between 2,400 m and 2,450 m. Their length varies between 800 m and 20 m, the width — between 375 m and 10 m, the area — between 1 m and 212 decares, the depth — between 0.5 m and 37.5 m. Most of them are between 2 m and 10 m deep and four reach depth of over 20 m. The water is transparent as deep as 15 m. Most of the lakes are covered with ice during most of the year — from October to June. The ice thickness reaches 3 m in the highest lakes. The largest glacial lake in Rila and in the Balkans is Smradlivo with an area of 212 decares; the longest is the Upper Ribno Lake reaching 801 m; the deepest