(ārnän'dō dā älärkōn') , fl. 1540, Spanish explorer in the Southwest. He was given command of a fleet that was supposed to support the land expedition of Francisco Vásquez de Coronado. In the summer of 1540 he sailed up the Gulf of California; proved definitely that Lower California was a peninsula, not an island; and discovered the Colorado River. He failed, however, to make contact with Coronado's expedition. He explored the river a few months before García López de Cárdenas discovered the Grand Canyon.