There has been understandable confusion about use of the plural Californias by Spanish colonial authorities. California historian Theodore Hittell offered the following explanation:
In 1767, the Jesuits were expelled from the missions, and Franciscans were brought in to take over. Gaspar de Portolá was appointed governor to supervise the transition. At the same time, a new visitador, José de Gálvez, was dispatched from Spain with authority to organize and expand the fledgling province. The more ambitious province name, Las Californias, was established by a joint dispatch to the King from Viceroy de Croix and visitador José de Gálvez, dated January 28, 1768. Gálvez sought to make a distinction between the Antigua area of established settlement and the Nueva unexplored areas to the north. At that time, almost the only explored and settled areas of the province were around the former Jesuit missions but, once exploration and settlement of the northern frontier began in earnest, the geographical designations Alta and Baja gained favor. The single province was divided in 1804, into Alta California province and Baja California province. By the time of the 1804 split, the Alta province had expanded to include coastal areas as far north as what is now the San Francisco Bay Area in the U.S. state of California. Expansion came through exploration and colonization expeditions led by Portolá, his successor Pedro Fages, Juan Bautista de Anza, the Franciscan missionaries and others. Independent Mexico retained the division but demoted the former provinces to territories, due to populations too small for statehood.
Department of Mexico
In 1836, the designation Las Californias was revived, reuniting Alta and Baja California into a single departamento as part of the conservative government reforms codified in the Siete Leyes. The Seven Laws were repealed in 1847, during the Mexican–American War, and the split of the two Californias was restored.
Following Mexico's defeat in the war, most of the former Alta California territory was ceded on 2 February 1848 to the United States, under the terms of the Treaty of Guadalupe Hidalgo. The new Mexico–United States border was established slightly to the north of the previous Alta-Baja border, and the terms Las Californias and Alta California were no longer formally used. The areas in North America acquired by the U.S. were designated as unorganized "territory" under a military governor, pending re-establishment of civilian control and organization. California was the first section of the territory to achieve statehood, two years later.
Geography
The Baja California Peninsula is bordered on three sides by water, the Pacific Ocean and Gulf of California ; while Alta California had the Pacific Ocean on the west and deserts on the east. A northern boundary was established by the Adams–Onís Treaty of 1819. That boundary line remains the northern boundary of the 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 as the areas to the west of the Sierra Nevada and the lower part of the Colorado River in the Lower Colorado River Valley.