[T]he maximum discharge known outside the period of record was about 8500 m³sec¯¹ on July 7, 1884. According to E. C. LaRue (1925), during this flood, a resident of Lees Ferry rescued his cat from the branches of an apple tree. Decades later, the resident, with “the height of the water on the trunk of the tree … well impressed on his mind,” assisted a surveyor in referencing that elevation to the datum of the stream gage established at Lees Ferry in 1923.
8,500 cubic meters per second, translates to about 300,000 cubic feet per second. That is a lot of water. Sustained over a single day, it would be enough to raise Lake Mead by more than five feet and meet Las Vegas Nevada’s demand for water for two years.
The quote is from O’Connor, Jim E., et al. “A 4500-year record of large floods on the Colorado River in the Grand Canyon, Arizona.” The Journal of Geology (1994): 1-9. I thank Scott St. George for bringing this to my attention.