The Lonquén River is in the Ñuble Region in the southern part of Central Chile. It forms at the confluence of the "esteros" Pichimávida and Colomávida that originate in the mountains in the east of the province. Its course lies from north to south, receiving from the west the flow of the "esteros" of La Huerta, Coipuencillo, El Salto, Coroney, and Ninhue, and from the east tributaries, "estero" Torrecilla, and other lesser tributaries like the "estero" Tiuque. The Lonquén then changes direction moving from east to west. In this section, it receives from the north the tributaries of the "esteros" Ranquil, Cholchol, Reloca, San Jose, and Santa Rosa, and from the south the "estero"s Corontas, Buenos Aires, Luengos, Cabrería, Molino, and Antequereo. It then empties into the Itata River from its northern bank as the Itata is turning toward the west.
Fort Lonquén was built in 1602 by the Governor Alonso de Ribera on the south bank of the Lonquén River 9 km above its confluence with the Itata River.