The Pirinska Bistritsa or Mpístritsas (Bulgarian: Пиринска Бистрица, Greek: Μπίστριτσας) is a river in south-western Bulgaria and northern Greece, a left tributary of the Struma. The river is 53 km long; the last 7 km before its confluence with the Struma forms the border between Bulgaria and Greece. Pirinska Bistritsa drains the south-western sections of the Pirin mountain range and the northern sections of the Slavyanka mountain range.
The river springs from the southern shore of the Argirovo glacial lake in situated in the Demirkapia cirque in Northern Pirin at an altitude of 2,365 m. It flows in southern direction in a deep valley. Near Pirin refuge it turns to the southeast and enters a densely forested area, where it again flows southwards. At the village of Pirin the river takes a sharp turn to the southwest and enters a sparsely forested valley with eroded slopes. At the village of Katuntsi it enters the Sandanski–Petrich Valley, where the river bed widens. At half a kilometer south of the village of Novo Hodzhovo is the confluence with the river Petrovska reka. From that point it forms the border between Bulgaria and Greece until its confluence with the Struma at an altitude of 69 m between the villages of Kulata and Promachonas.
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