It rises in the hill country east of Badulla and flows north and east past Inginiyagala to the Indian Ocean 10 miles (16 km) south of Kalmunai. The Gal Oya river is the main source feeding the Gal Oya scheme, a government program that dammed this and smaller rivers to create Senanayake Samudra—the largest tank (reservoir) in Sri Lanka, at Bintenne (Ampara). The Gal Oya National Park (founded 1954). The area is rich in biodiversity.

Senanayaka Samudraya;

FULL SUPPLY DEPTH ( Ft ) - 110.0

GROSS CAPACITY (Ac ft) - 770,000