A streamflow frequency curve is compiled from recurrence intervals computed for an annual series of streamflow data, ranked in order of magnitude. Peak-streamflow frequency (also known as flood frequency) curves are prepared from the annual series of maximum instantaneous peak discharge, and low-flow frequency curves are prepared from an annual series of all consecutive n-day periods of minimum daily mean flow, where n is the number of days, commonly seven.
A streamflow frequency statistic is a point on the streamflow frequency curve representing the magnitude of streamflow for a given recurrence interval. For example, the 50-year recurrence interval flow, or 50-year flood, is the magnitude of the peak streamflow expected to be exceeded once, on average, in 50 years. Streamflow frequency is perhaps better understood as an exceedance probability, which is the reciprocal of the recurrence interval. Every year, the probability that the 50-year flood will be exceeded is 1 in 50, or 2 percent.