The Mompóx Depression is found within this basin, this depression runs from the Zapatosa Marsh to the delta of the Magdalena River. This depression collects the most water in the department as its where the Cauca River, Cesar River, and the San Jorge River drain its waters. The Magdalena River is by far the most affluent river of these, and during the rainy season when it overflows, it causes the other rivers to flow backwards, not before overflowing the marshes and sometimes flooding some valleys.