What is the longest river that flows entirely within Colombia?

The Magdalena River is the longest river that flows entirely within Colombia.

The Magdalena runs for about 1,528 kilometres from the Andes to the Caribbean Sea. It rises near the southern Colombian Andes, passes between the Central and Eastern Cordilleras, and reaches the Caribbean near Barranquilla through a broad river system.

For centuries, the river has been Colombia’s principal north–south transport corridor. Spanish colonial settlements, ports, agriculture, and later industrial centres developed along its valley. The river has also been central to Colombia’s economy because it connects inland regions with Caribbean shipping routes.

The Cauca River is the Magdalena’s principal tributary and is also entirely Colombian, but it is shorter. The Putumayo and Caquetá belong to the Amazon drainage and cross or form international boundaries, so they do not fit the question’s wording.