Pirin forms the water divide between the basins of the rivers Struma and Mesta. The watershed follows the main ridge of the mountain in direction north-west to south-east. As the main ridge is situated closer to the Mesta valley, the tributaries to the Struma tend to be longer. The 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.