Ever since large dams and reservoirs were first constructed, extreme flood estimates have been produced for use in designing spillway capacities. For this purpose, the National Weather Service, the Corps of Engineers, and the Bureau of Reclamation have codified a methodology to estimate an extreme flood, which is commonly referred to as the Probable Maximum Rainfall and resulting Probable Maximum Flood event. The combined method uses historic data to extrapolate meteorologic and hydrologic conditions to extreme levels to produce the maximum flood estimates. As more data are collected, the estimates of the higher limits of extreme floods have increased.
If you can't find a tool you're looking for, please click the link at the top of the page to "Go to old article view". Alternatively, view our Knowledge Base articles for additional help. Your feedback is important to us, so please let us know if you have comments or ideas for improvement.