Part 2: Flood Frequency Curves Since large flood events are random, it is very difficult to predict flooding. What we can do, however, is use data from past floods on a river to calculate the probability of a flood occurring. Small floods have a high probability and happen almost every single year, but big floods with a low probability occur less often. The term "100-year flood" means that a flood that large has a 1% probability of occurring in any given year. The term "100-year flood" does not mean that a flood of that size occurs only once in a hundred years, just that the probability of a flood that size is very low. The United States Geological Survey (USGS) monitors discharge at gauging stations along many rivers to determine flood frequency and recurrence interval. Estimates of flood frequency are more accurate when there are many years of discharge records. The flood frequency is typically expressed as a recurrence interval. This is the average number of years expected between floods of a given magnitude, and it is calculated by listing all of the floods that have ever occurred and ranking them from the largest to the smallest. The USGS has