Stream Recurrence intervals: An important question for hydrologists, urban planners, land-use experts, and home owners to consider is "how frequently does a river overflow its banks and cause a flood?" The bar graph below of discharge values shows that the Gunnison River (1975-1995) reached a 3600 cfs discharge rate in 1990. It's important to know how often this rate will occur. Or, how often can we expect a much more serious flood event of perhaps, 6000 cfs?

The answer to these questions involves simple statistics that begin with the idea of the Recurrence Interval (RI) of a flood. RI is the average number of years between peak discharge values of a certain size. For example a river may overflow its banks on an average of once in 30 years with a discharge of at least 6000 cfs. Thus, the Recurrence Interval (for a 6000 cfs flood) is 30 years. It should be pretty obvious to you that in order to determine RI for different sized floods, we need to examine a stream's discharge over many years.