How is flood frequency analysis performed?

Flood frequency analysis estimates flood magnitudes for various return periods: (1) Collect annual maximum series from stream gauge records, (2) Rank data and assign plotting positions (Weibull, Gringorten), (3) Fit probability distribution (Log-Pearson Type III, Gumbel, Log-Normal) using method of moments or maximum likelihood, (4) Use fitted distribution to estimate flood quantiles for design return periods. Regional analysis combines nearby stations to improve estimates. Results subject to uncertainty from limited record length and non-stationarity (climate change).