How was the border between Nevada and California determined? Basically, it's an imaginary line drawn in the sand.
-- Peter Torrance, San Diego
For an imaginary line, it sure caused a ruckus. When Spain and Mexico owned the area known as California (Alta and Baja), there was no real boundary. I guess you just rode east until somebody shot at you and told you to get off his land, and that's where you drew the line. But once the U.S. took most of the West from Mexico in 1848, and the major cities in California wanted to petition Washington to become an official state, it was time to decide exactly what shape the state would be.
At the constitutional convention in Monterey in 1849, one faction wanted to scoop up all of the Utah Territory and draw the boundary somewhere east of Salt Lake City. Opponents argued that Washington would never admit such a greedy state, and perhaps a line east of the Sierras was more modest and realistic. I haven't found reports of actual fistfights, but the state boundary issue was nearly a deal breaker at the convention. Everybody threatened to take their votes and go home. The east-of-the-Sierras crew finally convinced the other side that a state including all the Utah Territory would be pretty hard to govern. Also, nobody from the Utah Territory had been invited to the convention, and they might not like to wake up one day and find out they suddenly lived in California. Besides, Mormons in Salt Lake were already typing up their own petition to Washington for the state of Deseret.
There was less controversy about drawing the actual line. The northern boundary was set along the 42nd parallel. The southern boundary had already determined by the Treaty of Guadalupe-Hidalgo with Mexico. The northeast corner of the state would be the point where the 42nd parallel met the 120th meridian, to include the Sierras. Follow the 120th parallel south to the point where it intersects the 39th parallel (Lake Tahoe), then draw a straight line south to the intersection of the 35th parallel and the Colorado River. The river would form the remainder of the boundary to the border with Mexico. Everybody was satisfied, they went home, Washington pondered the proposal, and California became a state in 1850.