Here are some facts and insights that help us understand what the American colonists might have known, and how both the knowledge of and interaction with California increased over time.
The Spanish were a major force among the European powers in the Age of Discovery. By 1520 the Spanish laid claim to over half of the known world, including most of the Americas. However when you review actual settlements by the Europeans in the new world, Spain’s development was most extensive in the southern half of the Americas. The Spanish focus in what is today the United States started with the exploration of Florida in 1513.
[inline:spanishroutemap.gif=Spanish Exploration of Florida - 1513]
The first successful permanent settlement in the United States was St. Augustine, Florida, founded by the Spanish in 1565. The Spanish tried to expand up the coast. They established a fort on St. Catherine’s Island, south of present day Savannah, Georgia in 1566. They even sent missionaries into present day Virginia in 1570 but this settlement was overrun by Indians within a few weeks.
While the English had an early claim to this land, they did not begin to settle parts of present day United States until much later than the Spanish. Their first attempt at the settlement of Virginia was in 1584, a decade and a half after the Spanish. The first permanent settlement (at Jamestown) was in 1607. Massachusetts wasn’t settled by the Pilgrims until 1620. Charlestown was settled in 1670. During much of this period there was conflict between the English and the Spanish. When the English, led by James Oglethorpe, settled Georgia (Savannah in 1733 and Augusta in 1736) conflicts between the English and Spanish accelerated, and continued for decades.
So in part the answer to your question about the knowledge of the American Colonist about the Spanish possessions depends on whether you were a colonist in New England (where the major conflicts were with the French) or Georgia.
During much of early U.S. Colonial history California was still a wilderness. It was only founded in 1769 less than a decade before the Declaration of Independence. There were maps showing (often inaccurately) the territory of California. The more scholarly among the colonists would have had some idea of Spanish territorial claims and known settlements. However the vast majority of the colonists would have focused on the struggles for territory that affected them most directly.
I asked Dr. Robert H. Jackson, a noted borderlands scholar for his perspective on what the early American colonists might have known about California. Here is his commentary:
“At the time of the American Revolution English colonists /Americans knew very little about California, other than the fact that it existed. American ships did not begin to visit the coast of California until a decade or so after the American Revolution, in the 1790s and early 19th century. Moreover, the Spanish government did not publish information about California. The first travel accounts such as Captain George Vancouver's A Voyage of Discovery did not appear in print until much later (1798). Maps in the 18th century did include California. But there was a time lag of sorts in the incorporation of information.
Dr. Jackson sent me the map which shows below. It was published in England in 1763, the year in which the Spanish lost St. Augustine to the English.
[inline:namap1763.jpg=Map of North America - 1763]
As the new country of the United States expanded, knowledge of the original Spanish territories in the southwest - what the Spanish called their Northern frontier - increased. By 1821 this entire area (extending from Texas into California) was part of the new country of Mexico. American settlers pushed into Texas in large numbers in 1820s. American trappers explored much of the southwest and west coast in these years. One of my favorite Mountain Men, Jedediah Strong Smith, visited the California missions in 1826-27. Ships from New England were regularly trading with the missions by the 1820s, as Dr. Jackson has noted. While sine Americans settled in California during the Spanish era, the biggest influx came in 1840s when California was part of Mexico, as overland routes opened up and General Mariano Vallejo encouraged American immigration.
Ultimately it was the conflict known as the Mexican American War (1846-48) that led to the inclusion of California into the United States. In the Treaty of Guadalupe Hidalgo (1848) Mexico ceded 55% of its territory to the United States (the territory included present-day Arizona, California, New Mexico, Texas, and parts of Colorado, Nevada and Utah).