On the right is the data you prepared for annual peak discharge (Qmax) on Colorado's Gunnison River. The Qmax values are a little different than your because these Q values are the exact ones taken from the United States Geological Survey "real-time data" web site.

In order to calculate the Recurrence Interval (RI) of a flood of a certain size we need to first RANK-ORDER the discharge values from greatest to least. The maximum peak flow (3620 cfs) happened in 1990, so its rank is #1, whereas the minimum peak discharge (109 cfs)occurred in 1979, so its rank is #21 (there are 21 years of data). Fill in the rest of the rank values. (Some Qs are rank-ordered for you.)

The Recurrence Interval (RI) is calculated as follows:
RI = (n +1) ÷ Rank, where n is the # of years of data (21 in this case).

The peak flow of 1990 has rank = 1, so its RI is (21 + 1)/1 = 22 years.

Calculate the RI for the 21 years of data. (Some RI values have been determined for you.)