[1] Under the name Manastirska River, it takes its source from the north-eastern corner of the Upper Ribno Lake in the western part of Central Rila at an altitude of 2,225 m in a cirque surrounded by the peaks Yosifitsa (2,697 m) in the east, Kanarata (2,666 m) in the south and Kyoravitsa (2,612 m) in the west.

[1] There, the Rilska River turns in south-western direction and after 10 km flows into the Struma at an altitude of 346 m in the outskirts of the town of Kocherinovo.

[1] Its left tributaries have more contribution to the annual flow because they are situated in southern end of the valley on slopes that face in northern direction and accumulate more snow.

[1] The entire length of 38 km of the third class road III-107 between Kocherinovo and the Rila Monastery follows the Rilska River valley.

On the river's left banks some 4.8 km downstream of the confluence with the Iliyna is located the Rila Monastery, one of Bulgaria's oldest and most important sanctuaries.