Great question- I was looking at highest daily average, which was 10,900 CFS- using the method that @marksjeep provided, the highest instantaneous value for the Stillwater was June 15, 1967 when it hit 12,000 CFS. So 23,900 CFS nearly doubled the recorded all time instantaneous max flow, assuming the meter was accurate and operating correctly.Is that all time high for that particular day or the highest ever recorded? People (including me) were thinking that the max value on the USGS site for the Yellowstone (30,000 cfs) was the highest ever, but it is actually the highest on that particular day.
If you look at the history you can see Yellowstone flows even touched 40,000 cfs one year, but that is still much less than the 50,000 cfs yesterday.