The territory of Pirin National Park is almost equally divided between the basins of the rivers Struma (206.06 km2 or 51.1%) and Mesta (197.50 km2 or 48.9%). The watershed follows the main ridge of the mountain in direction north-west to south-east. Pirin is the source of 10 tributaries to the Struma, the largest one being Sandanska Bistritsa, and another 10 to the Mesta. The rivers are short, steep and with high water volume. They form numerous waterfalls which are generally not as high as those in Rila or the Balkan Mountains. The highest one is Popinolashki waterfall, measuring some 12 m. The average annual discharge of the park's rivers is 355,6 million m3, of them 188,5 million m3 flow to the Struma and 167,1 million m3 to the Mesta. The discharge from the park per square kilometre is 2.3 times larger than Pirin's average and 5.6 times larger than Bulgaria's average.