Flooding in the Mississippi River Basin in Minnesota, Spring 2001

During spring 2001 there was much flooding in the Mississippi River Basin in Minnesota. Greater than normal precipitation starting with late fall rains in 2000, greater than normal snowfalls, a delayed snowmelt, and record rains in April, all contributed to the flooding. Parts of the southern one-half of Minnesota had streamflows of magnitudes not seen in more than 30 years. Approximately 50 counties were declared disaster areas with greater than 34 million dollars in total reported flood damage (S. Neudahl, Department of Public Safety, Division of Emergency Management, oral commun. July 9, 2001). Record flows were recorded at nine U.S. Geological Survey (USGS) continuous-streamflow gaging stations within Minnesota or along the Wisconsin border. In addition, 14 stations recorded their 2nd or 3rd highest peaks of record. These gaging stations are maintained by the USGS as part of a network of gages used by the USGS, U.S. Army Corps of Engineers, National Weather Service, Minnesota Department of Natural Resources, as well as other State and local agencies. Stage and discharge data gathered at these gages are used for flood forecasting, operations of dams and diversions, managing emergency activities during floods, and performing follow-up studies to better understand river flood-flow characteristics. This report summarizes flooding in that part of the Mississippi River Basin that includes stream-gaging stations within the borders of Minnesota and monitoring points along the Minnesota-Wisconsin border. Peak stages and discharges are included in this report for 44 stream-gaging stations (table 1) maintained by the USGS in the Mississippi River Basin. Stage, in feet, is based on an arbitrary scale established when a gaging station is installed. Discharges are instantaneous determinations of peak flow, in cubic feet per second. This peak flow information is ranked with the five greatest discharges of record for each station. In addition, flood-recurrence intervals are included for stations with at least 20 years of record, and/or with little influence on peak flows by regulation. Flood-recurrence intervals are expressed in years and are the probability of a flood of this magnitude occurring. A 25-year event can be expected to occur, on average, once every 25 years. This probability is independent of individual events, so two '25-year' floods can occur in successive years. It is also possible for the probability value assigned to a flood (for example, greater than 25-year event) to change over time. As peak flows are included from subsequent years, flood frequencies previously determined can change depending on the magnitude of these additional flows. This is a primary reason why flood frequencies are not included in this report for stations with less than 20 years of record. Even with 20 or more years of record, significant changes to previously established flood frequencies are possible. This report presents preliminary water-resources 2001 flood data obtained from streamflow-gaging stations in the Mississippi River Basin (fig. 1).