Catastrophic flash flooding occurred in the early morning hours of June 11, 2010, in the upper Little Missouri River and tributary streams in southwest Arkansas. The flooding, which resulted in 20 fatalities and substantial property damage, was caused by as much as 4.7 inches of rain falling in the upper Little Missouri River watershed in 3 hours. The 4.7 inches of rain in 3 hours corresponds to estimated annual exceedance probability of approximately 2 percent for a 3-hour duration storm. The maximum total estimated rainfall in the upper Missouri River watershed was 5.3 inches in 6 hours. Peak streamflows and other hydraulic properties were determined at five ungaged locations and one gaged location in the upper Little Missouri River watershed.The peak streamflow for the Little Missouri River at Albert Pike, Arkansas was 40,100 cubic feet per second, estimated to have occurred between 4:00 AM and 4:30 AM the morning of June 11, 2010. The peak streamflow resulted in average water depths in the nearby floodplain (Area C of the Albert Pike Campground) of 7 feet flowing at velocities potentially as great as 11 feet per second. Peak streamflow 9.1 miles downstream on the Little Missouri at the U.S. Geological Survey streamgage near Langley, Arkansas was 70,800 cubic feet per second, which corresponds to an estimated annual exceedance probability of less than 1 percent.