The USGS stream gage on the Virgin River near Littlefield, Arizona peaked at about 27,000 cfs on Dec. 20, up from its typical flow of about 100 cfs. That explains the damage and destruction of homes along Beaver Dam Wash.
There are a couple dozen videos on YouTube of flooding on the Virgin nearby around St. George, Utah.
Thanks to John Fleck at Inkstain for posting the graph. He talks about the water flow into Lake Mead as well.
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