rivers are short, steep and with high water volume; the river bottom is rocky or covered with large sediments. The water discharge is highest in May and June and lowest in September. The streamflow is largely derived from snow melting and glacial lakes in spring and summer and almost exclusively from groundwater in winter. The rivers and streams form numerous leaps and waterfalls but they are generally not as high as those in Rila or the Balkan Mountains. The highest one is Popinolashki waterfall, measuring some 12 m. Pirin is the source of 10 tributaries to the Struma, including Pirinska Bistritsa, Sandanska Bistritsa, Melnishka reka and Vlahina reka, as well as another 10 to the Mesta, such as Iztok, Disilitsa and Retizhe. Reaching a length of 53 km, Pirinska Bistritsa is the longest river in the massif. Pirin has abundant mineral springs, being among Bulgaria's richest areas. Based on the chemical composition they are divided into two thermal groups — in the Mesta and the Struma basins. The springs in the Mesta basin to the east are more alkaline, with lower mineralisation (below 400 mg/L) and conductance; those on the western slopes reach higher temperatures, up to 86 °C. 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: Popovo Lakes – 11 Kremenski Lakes – 5 Banderishki Lakes – 16 Vasilashki Lakes – 12 Valyavishki Lakes