Bío-Bío (bē´ō-bē´ō), river, c.240 mi (390 km) long, rising in the Andes of central Chile and flowing NW to the Pacific Ocean near Concepción. It forms a natural divide between middle and southern Chile. It is navigable for much of its length by flat-bottomed boats, but is rapid and has hydroelectric potential. In colonial times bitter fighting took place along its banks between Spanish forces under Pedro de Valdivia and Araucanian natives. In 1612 the Bío-Bío was fixed as the boundary to native territory; today the Mapuche people live to the south.