First European Explorer of the Colorado River: Francisco de Ulloa

The explorer who first visited the Colorado River—the largest river by discharge within its U.S. state (Arizona) and a natural boundary with Mexico—was the Spanish navigator Francisco de Ulloa.

The Expedition (1539–1540)

Commissioned by Hernán Cortés, Ulloa led a fleet of three ships up the Gulf of California (Sea of Cortés) in 1539. He sailed to the head of the gulf, proving Baja California was a peninsula, not an island. In 1540, he entered the mouth of the Colorado River and sailed upstream for a considerable distance, becoming the first European to document its course and the powerful tidal bore at its mouth.

Significance

• Created the first European map of the Colorado River delta and lower course.
• Confirmed the river's connection to the Gulf of California.
• His reports informed later expeditions (Alarcón, Díaz, Oñate) and Spanish understanding of the region for centuries.

While later explorers like Hernando de Alarcón and Melchior Díaz explored further upstream in 1540, Ulloa was the first to reach and chart the river from the sea.