The Mississippi River is the largest river system in the United States of America. It flows entirely in the United States, through thirty-one states, and up into Canada. The Mississippi ranks as the fourth longest river, just after the Nile, the Amazon, and the Yangtze, and tenth largest among the world's largest rivers.
Percy Jackson fights Echidna and the Chimera at the Gateway Arch. When the Chimera poisons Percy he jumps into the polluted waters of the Mississippi River. He then discovers some of his water powers such as breathing underwater and keeping things dry, when a Nereid tells him to go to Santa Monica Beach.