The megaflood was a result of sustained major rain storms across the region, enhanced by an unusually powerful atmospheric river.
[1] The flooding affected the indigenous peoples of California, in pre-industrial advancement populations.
In addition to this event, geologic evidence indicates that other "megafloods" occurred in the California region in the following years A.D.: 212, 440, 603, 1029, c. 1300, 1418, 1750, 1810, and 1861–1862.
[2][1] United States Geological Survey sediment research revealed that the 1605 flood deposited a layer of silt two inches thick at the Santa Barbara basin, indicating that it was the worst flood event of the past 2,000 years, being at least 50% more powerful than any of the others recorded based on geological evidence.
[1] The United States Geological Survey has developed a hypothetical scenario, known as the "ARkStorm", that describes the effects of a similar event in modern-day California.