California’s southern border was resolved in 1848 by international treaty after the Mexican-American War, but until 1849 its northern limits stretched into what was known as Oregon Territory, and its eastern border extended, somewhat vaguely, all the way into present-day Utah.
The 1849 pre-statehood Constitutional Convention at Monterey set the 42nd parallel as the line between California and Oregon, but for several days the delegates couldn’t agree on an eastern state line. After much controversy they agreed that the Sierra Nevada (shown in blue) formed a natural boundary and decided to set the state’s eastern border on the east side of the mountains—but this meant that a single, north-south line wouldn’t be suitable. Therefore, the delegates chose to draw a straight line south from the 42nd parallel that passed through Lake Tahoe, and then turned in an oblique southeast angle until it met a natural, if irregular, boundary along the Colorado River.
While this established the state’s odd shape, unfortunately the Convention delegates failed to authorize a geographic survey—and river channels change over time. The result was aggravation and litigation between California, Nevada and Arizona for more than a century. The Arizona-California Boundary Commission met in 1943 to establish points of longitude and latitude to correct confusion created by meanderings in the main channel of the Colorado River. After several conflicting surveys over the years, the United States Supreme Court settled the California-Nevada boundary issue in 1980.