Ferdinand de Soto
"In the spring of 1541, six years from the discovery of the river St. Lawrence, another equally important river, the Mississippi, was discovered. This honor belongs to Ferdinand de Soto, a Spaniard, who, having projected the conquest of Florida from the natives, arrived from Cuba in 1539, with a considerable force. He traversed the country to a great distance, and in the spring of 1541, first discovered the Mississippi, five or six hundred miles from its mouth.
The object of Soto, in traversing so wide an extent of country, appears to have been to search for gold. The summer and winter of 1539 he spent in Florida. In 1540, he began his tour northeast, and, having crossed the Altamaha, Savannah, and Ogechee rivers, he turned westerly, and crossing the Alleghanies, proceeded southwardly as far as Mobile and Pensacola. The winter of this year he spent with the Chickasaws. The following spring, he made the important discovery above mentioned.
The next year in 1542, Soto died on the banks of the Mississippi river, May 21st, in the bosom of whose waters he was buried. Under the guidance of successor whom Soto had appointed, his followers wandered about the country, in an ineffectual effort to penetrate Mexico. During these wanderings, they once more came upon the Mississippi, a short distance above Red river. Here they encamped, and proceeded to build several large boats, on which they embarked, July 12th 1543, and in seventeen days reached the Gulf of Mexico. Continuing their voyage, in the following September they reached a Spanish settlement at the mouth of the river Panuco, in Mexico."
— A History of the United States, by Charles A. Goodrich, 1857 (edited)