river in south-central Chile. It rises in the Icalma and Galletué lakes in the Andes on Chile's eastern border and flows generally northwestward to enter the Pacific Ocean near Concepción after a course of 240 miles (380 km). After crossing the fertile Central Valley, it forms the only major transverse valley through the coastal mountain range. Although one of Chile's longest rivers, it is navigable only by flat-bottomed boats. The Bío-Bío is commonly regarded as the southern limit of middle Chile, where about three-fourths of the country's people live.