Sandanski–Petrich Valley in the context of Strumica (river)

Petrich (Bulgarian: Петрич [ˈpɛtrit͡ʃ]) is a town in Blagoevgrad Province in southwestern Bulgaria, located in Sandanski–Petrich Valley at the foot of the Belasica Mountains in the Strumeshnitsa Valley. According to the 2021 census, the town has 26,778 inhabitants.

It is the seat of Petrich Municipality.

Sandanski (Bulgarian: Сандански [sɐnˈdanski]; Greek: Σαντάνσκι, formerly known as Sveti Vrach, Bulgarian: Свети Врач, until 1947) is a town and a recreation center in southwestern Bulgaria, part of Blagoevgrad Province. Named after the Macedonian Bulgarian revolutionary Yane Sandanski, it is situated in Sandanski–Petrich Valley at the foot of Pirin Mountains, along the banks of Sandanska Bistritsa River. Sandanski is about 20 km away from the Bulgaria-Greece border and 100 km away from the Aegean Sea.

The town has a convenient location, a mild to warm climate (with the highest average annual temperature in the country, +16°C) and relatively high concentration of thermal water springs, which all make it a popular destination for relaxation and recreation.

The Sandanska Bistritsa (Bulgarian: Санданска Бистрица) is a river in south-western Bulgaria, a left tributary of the Struma. The river is 33 km long and drains the south-western sections of the Pirin mountain range.

The Sandanska Bistritsa takes its source from the south-eastern corner of the Tevnoto Lake in Northern Pirin at an altitude of 2,512 m. It flows in southern direction through another four lakes of the Malokamenishki Lakes group and then turns westwards. After the confluence with the first significant right tributary, the Bashliytsa, the turns to the south-west and flows in a deep sparsely forested valley. In the upper course of the river is located the Popinolashki waterfall, one of Pirin's largest. Near the town of Sandanski the river enters the Sandanski–Petrich Valley forming a large mucous cone. It flows into the Struma at an altitude of 104 m near the Sandanski industrial zone.

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.