The earthquake of May 25, 1751 devastated the city and changed the main course of the riverbed towards the so-called "La Lajuela estuary", which is where it runs today, leaving the old channel gradually dry, abandoned and disconnected from the main one, becoming known as the Río Viejo estuary, like the sector where the bifurcation into two arms occurred.
On its banks, near Pinto, the battle of Vegas de Saldías took place, which put an end to the guerrilla movement of Vicente Benavides, in the period known as the War to the Death.
It runs west along a bed that is initially surrounded by the wooded heights of the Andes foothills; it passes near the north of the city of Pinto, opposite which there is a bridge built in 1889 on the road to Coihueco.
It enters the central valley or plain, where it begins to lose its former speed and continues along a low, extended bed until it approaches the city of Chillán Viejo, whose southern side it bathes, and then divides into two branches, the main one continuing towards the SW along a narrow, deep channel or ravine that it has cut through since the end of the last century and over which there is another bridge.
It is soon reunited into one body and increased by the small streams of Cadacada, Bollén and Quilmo, which it receives on its left, and the Maipón, on its right, it flows towards the NW.