The man who explored the Mississippi River was Rene-Robert Cavelier, Sieur de la Salle. La Salle, the son of a wealthy merchant, came to the New World to set up trade with the Native Americans.
La Salle carried out a number of expeditions around the Great Lakes and tributaries. When La Salle discovered the Mississippi River, he claimed it and its surrounding watershed Louisiana after King Louis XIV of France. After the initial expedition, La Salle made a trip a few years later with four ships to try to find the mouth of the Mississippi and set up a colony. One of the ships was taken by pirates. In the attempt to reach the Gulf, one ran aground and all the cargo was lost, one was taken by its captain back to France and the other stayed with the soon to fail colony.