To evaluate the effects of the 1993 flood in the upper Mississippi River Basin on the determination of flood magnitude and frequency, discharges that had recurrence intervals of 10, 25, 50, and 100 years computed from data through the 1992 water year were compared with those computed from data through the 1993 water year for 62 selected streamflow-gaging stations in Iowa. On the basis of the flood-frequency analysis computed from data through the 1993 water year, a flood that was greater than or equal to a 10-year recurrence interval discharge occurred during 1993 at all 62 gaging stations, and a flood greater than or equal to a 100-year recurrence-interval discharge occurred at 11 of the gaging stations.

Results of the comparison indicated that inclusion of the 1993 flood in the data base resulted in an increase in the magnitude of dis charges for all selected recurrence intervals at the 62 streamflow-gaging stations in Iowa. A larger percentage increase in the magnitude of discharge was computed for the larger recurrence intervals than for the smaller recurrence intervals for most of the selected gaging stations . As a result of including the 1993 peak discharge in the flood-frequency analysis, three gaging stations had an increase in the 100-year recurrence-interval discharge that was greater than 30 percent.

Several factors, which included recurrence intervals for the 1993 peak discharges and the effective record lengths for 1993, were investigated for the 62 selected streamflow-gaging stations to evaluate their possible effect on the computed flood-frequency discharges. The combined effect of these two factors on the computed 100-year recurrence-interval discharges was significant. Gaging stations were grouped into four discrete categories on the basis of recurrence intervals for the 1993 peak discharges and the effective record lengths for 1993 . Of the 28 gaging stations that had small flood magnitudes in 1993 and long record lengths, the difference between the 1992 and the 1993 flood-frequency analyses for 100- year recurrence-interval discharges at 22 gaging stations was less than 5 percent. Of the 10 gaging stations that had large flood magnitudes in 1993 and short record lengths, the increase in 100-year recurrence-interval discharges at 9 gaging stations was greater than 15 percent.