U.S. states of California, Nevada, and the western part of Utah. Inland regions were mostly unexplored by the Spanish, leaving them generally outside the control of the colonial authorities. Mountain ranges of the Peninsular Ranges, eastern Transverse Ranges, and the Sierra Nevada, along with the arid Colorado Desert, Mojave Desert, and Great Basin Desert in their eastern rain shadows, served as natural barriers to Spanish settlement. The eastern border of upper Las Californias was never officially defined under either Spanish or subsequent Mexican rule. The 1781 Instrucciones and government correspondence described Alta California ("Upper California") as the areas to the west of the Sierra Nevada and the lower part of the Colorado River in the Lower Colorado River Valley (the river forms the present day border between the states of California and Arizona). See also List of governors in the Viceroyalty of New Spain Spanish missions in Baja California Spanish missions in California Indigenous peoples of California Population of Native California Indigenous peoples of Baja California Ranchos of California History of California History of California through 1899 Territorial evolution of California Spanish colonization of the Americas The Canadas The Carolinas The Dakotas The Floridas The Virginias References Further reading External links Worldstatesmen.org: Provinces of New Spain Regions of North America Californias Californias Californias Californias Californias Californias Californias Californias Californias Californias Californias Californias