From the Introduction: "Flood-frequency analysis is a statistical technique used to predict the magnitude and frequency of peak flows in a stream. The objective of frequency analysis is to relate the magnitude of peak flows to their frequency of occurrence through a probability distribution. The probabilities computed correspond to the annual exceedance probability (AEP), or the probability in any year that a peak flow is exceeded. The flood-frequency analyses in this report, prepared in cooperation with Maine Department of Transportation, follow the methodology described in the current version of the national guidelines for flood-frequency analyses, hereafter referred to as 'Bulletin 17C' (England and others, 2018). A peak-flow frequency analysis is based on a statistical evaluation of annual maximum instantaneous flows collected at streamgages. Previous peak-flow studies in Maine and elsewhere have commonly described peak-flow frequency relative to recurrence intervals, which are the inverse of AEPs."
U.S. Geological Survey Scientific Investigations Report 2020-5092
U.S. Geological Survey: https://www.usgs.gov/