A total of 176 glacial lakes are situated in the mountain range, all of them in the North Pirin. Of them, 119 are permanent, and the rest dry out in summer. About 65% of them are located in the north-eastern area and discharge into the Mesta basin; the other 35% are to the south-west, their water flowing into the tributaries of the Struma. Approximately 90% of the lakes are nestled between 2100 m and 2450 m. At an altitude of 2710 m the Upper Polezhan Lake is the highest in Pirin, as well as in Bulgaria and the Balkan Peninsula. The total area of all lakes is 18 km2; the largest one being Popovo Lake with 123,600 m2, which makes it the fourth largest glacial lake in the country. With a depth of 29.5 m, it is also Pirin's deepest lake and Bulgaria's second. The water is transparent as deep as 15 m. Many of the lakes are covered with ice during most of the year. The ice thickness reaches 3 m in the highest lakes. The lakes and the lake groups are: