Sakarya (säkäryäˈ) [key], anc. Sangarius, river, c.490 mi (790 km) long, rising on the Anatolian plateau, NW Turkey. It flows generally north in a series of huge bends past Adapazari to the Black Sea at Karasu. There are hydroelectric-power plants on the river. The Porsuk and Ankara rivers are its chief tributaries.