Cachapoal River is a tributary river of the Rapel River in Chile located in the Libertador General Bernardo O'Higgins Region. The river gives its name to the Cachapoal Province.

The valley takes its name from the Cachapoal River that flows through Rapel Valley along with its tributaries, the Claro and Cortaderal rivers. All these watercourses flow into Rapel Lake.

Cachapoal River begun to incise in the rising Andes in the Miocene epoch. Later, as glaciers developed in the Andes the upper part of the valley was glaciated and reshaped into a glacial valley.^[1]

The climate of the valley is temperate and consistently Mediterranean, sheltered by the coastal range from the cooling influences of the Pacific Ocean.