Which Spanish river system receives water from the southern side of the Pyrenees?

The Spanish river system that receives water from the southern side of the Pyrenees is the Ebro River.

The Ebro basin, located to the south of the mountain range, is fed by numerous tributaries originating in the Pyrenees, including the Aragón, Segre-Cinca, and Gállego rivers. These left-bank tributaries provide the overwhelming majority of the Ebro's total volume, which is significantly bolstered by snowmelt and rainfall from the high peaks.123