Balsas, Río, river, c.450 mi (720 km) long, rising in the state of Puebla, E central Mexico. One of Mexico's longest rivers, it flows in a curve from south to northwest through Puebla and Guerrero states, where it waters a fertile valley, to Michoacán state, forming most of the boundary between the last two states. Then it turns southwest, passing through a hot, dry region before emptying into the Pacific Ocean. It is also known as the Río Mescala.