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Spanish exploration and early settlement

Starting in the 1500s, the Spanish began to explore and colonize western North America. Francisco de Ulloa may have been the first European to see the river, when in 1536 he sailed to the head of the Gulf of California. In 1540 García López de Cárdenas became the first European to see the Grand Canyon, during Coronado 's expedition to find the Seven Cities of Gold ("Cibola"). Cárdenas was apparently unimpressed with the canyon, greatly underestimating its size, and left in disappointment with no gold to be found. In the same year Melchior Díaz explored the Colorado River's delta and named it Rio del Tizon ("fire brand river"), after seeing a practice used by the local people for warming themselves.

By the late 1500s or early 1600s, the Utes had acquired horses from the Spanish, and their use for hunting, trade and warfare soon became widespread among Utes and Navajo in the Colorado River basin. This conferred them a military advantage over Goshutes and Southern Paiutes that were slower to adopt horses. The Navajo also adopted a culture of livestock herding as they acquired sheep and goats from the Spanish. Juan Bautista de Anza in 1774 was the first Spaniard to reach Yuma Crossing, where he established friendly relations with the Quechan people and opened the Anza trail between Arizona and the California coast. The Spanish soon founded Mission Puerto de Purisima Concepcion and Mission San Pedro y San Pablo de Bicuner along the lower Colorado River. However, Spanish attempts to control the crossing led to the 1781 Yuma revolt, in which over 100 soldiers and colonists were killed, and the settlements were abandoned. The Quechan blocked foreigners’ use of the crossing until the arrival of American mountain men and fur trappers in the 1820s.

The name Rio Colorado first appears in 1701, on the map "Paso por Tierra a la California" published by missionary Eusebio Kino, who also determined during that time that Baja California was a peninsula, not an island as previously believed. In the 1700s and early 1800s many Spanish and American explorers believed in the existence of a Buenaventura River that ran from the Rocky Mountains to the Pacific coast. In 1776, Silvestre Vélez de Escalante attached this name to the upper Green River, and a number of later maps showed this connecting to Lake Timpanogos (now Utah Lake) and flowing west to California. The Dominguez–Escalante expedition first reached the Colorado River near the junction with the Dolores River, naming the larger river “Rio San Rafael”. They later forded the Colorado in southeastern Utah at Crossing of the Fathers, now submerged in Lake Powell.