The Bolgarchay (, ) is a river, which forms part of the AzerbaijanâÂÂIran border. It is long, and has a drainage basin of .
The Bolgarchay rises from slopes of QaracadaÃÂ, Talysh Mountains at the border of Iran and Azerbaijan. It is joined by its tributaries, all flowing from the Iranian side of its drainage basin. It flows along the Iranian-Azerbaijani border, and enters Azerbaijan at Bileh Savar. The Bolgarchay pours into . River's flow comes from precipitation and groundwater. In summer, river's flow decreases and sometimes dries up.
In 1965, a water reservoir was built on the Bolgarchay in Bilasuvar District. Reservoir has water volume of and active storage of , with area of . This reservoir enables watering area.
The Bolgarchay is depicted as a western border of Talysh Khanate in maps describing the Russo-Persian War (1804âÂÂ1813).
River was explicitly mentioned in Treaty of Turkmenchay as a part of the border between Qajar Iran and Russian Empire The Bolgarchay has since served as a northern border of Iran with Azerbaijan Democratic Republic, Azerbaijan Soviet Socialist Republic, and finally Republic of Azerbaijan. Now it marks the entire border between Jalilabad and Iran, and part of the border between Yardimli District and Iran.