Flow-duration curves are cumulative frequency curves compiled by ranking all daily mean streamflows for a given time period in order of magnitude, then computing the percentage of time each streamflow is equaled or exceeded. The curve is a plot of the streamflows against their respective exceedance probabilities.

A flow-duration statistic is a point on the flow-duration curve representing the percentage of time a particular streamflow is equaled or exceeded during a given period. For example, the monthly 98-percent duration flow for July, considered a low flow, is equaled or exceeded 98 percent of the days in July. The annual 2-percent duration flow, considered a high flow, is exceeded 2 percent of the days in the year.