 Despite this apparent abundance of water, water volume is unevenly distributed throughout the country.Indeed, five rivers—the Usumacinta, Grijalva, Papaloapán, Coatzacoalcos, and Pánuco—account for 52 percent of Mexico's average annual volume of surface water.All five rivers flow into the Gulf of Mexico; only the Río Pánuco is outside southeastern Mexico, which contains approximately 15 percent of national territory and 12 percent of the national population.