Can any changes in flooding patterns be documented? The U.S.G.S. Report used historical data (available from the U.S.G.S. surace water database for North Dakota) to analyze the frequency of flood events. The figure below is modified from the U.S.G.S. report and shows the Recurrence Interval in years along the horizontal axis vs. discharge in cubic feet per second along the vertical axis, for a gaging station at Fargo. The data are broken into four time periods: 1882-1910, 1911-1938, 1939-1966, and 1967-1994.
How is this plot interpreted? The Recurrence Interval (RI) is related to the probability of a flood discharge of a certain magnitude. The discharge that occurs at a Recurrence Interval of 50 years is the theoretical 50-year flood discharge. The probability of that discharge is 1/50, or 2 percent.
The plot shows that there has been large variation over time in flood frequency. The U.S.G.S., however, believes this variation is not greater than expected. For example, the data from the period 1882-1910 (plotted using circles) shows that the floods experienced at that time were of similar frequency and size as those in recent time periods.