Floods of great magnitude occurred on the Mississippi River and tributaries, and on streams in Minnesota tributary to Lake Superior, during the months of April to June 1950. They were caused by an unusual combination of hydrologic conditions. These were: high soil-moisture content, very heavy snow cover, delayed snow melt, and heavy precipitation. An outstanding feature of the floods in the upper portions of the Mississippi River basin in Minnesota was the prolonged period that the streams remained above flood stage. For example, at Fort Ripley the average duration of floods since 1907 has been 7 days; but in 1950 the river remained above flood stage for periods of 13 and 34 days with a very short interval of in-the-bank flow between. Snow surveys by the Corps of Engineers in mid-March indicated an exceptionally high water content in the vicinity of the Mississippi River headwaters. This survey showed a moisture content of more than 4 inches as far downstream as the Gull Lake Reservoir; in the Crow Wing River basin near Braiherd, the maximum was 5.09 inches. Information regarding the water content -of the snow pack in river basins tributary to Lake Superior is not available. The Weather Bureau reported that 72. 5 inches of snow lay on the ground on April 11 at International Bridge north of Grand Marais. At Duluth,. Mikn., new records were established for April snowfall and for the depth of snow on the ground so late in the season. Conditions in the northern portion of the Lake Superior basin were not as extreme as in the southern portion; therefore, outstanding floods were mostly confined to the St. Louis River basin. The crests at gaging stations near the headwaters of this basin were the highest since records began in 1942. The Minnesota Power & Light Co. reported that the flow at their Thompson plant near Carlton was the highest since the start of the record in 1908 and about 50 percent greater than the previous peak flow in 1948. In the Mississippi River basin new record-high discharges occurred at gaging stations on tributary streams down to the Crow Wing River, where the 1950 crest was slightly less than that of 1943. On the main stem, there were record-high discharges down to Anoka, Minn. , and at Prescott, Wis., just below the St. Croix River. In the St. Croix River basin, record-high discharges occurred at gaging stations on the main stem; the discharge at St. Croix Falls, Wis. , was the maximum since records began in 1908. The area of most spectacular flooding was in the vicinity of Aitkin, Minn., on the Mississippi River, where the stage was computed by the Corps of Engineers as being 2. 46feethigher than the previous maximum of 1888. The region was designated as a disaster area by the Governor of Minnesota. The Red Cross, National Guard, Coast Guard, and Corps of Engineers were the principals in relief work. The damage in the headwater area was estimated by the Corps of Engineers at more than $2,675,000. Estimates by the Corps of Engineers and the Weather Bureau indicate that the total damage in the area covered by this report may have exceeded $5, 000, 000. Only one life is known to have been lost as a direct result of the floods. During the last days of March, discharge on the Root River reached within 400 cfs of the maximum of record, which occurred in March 1933--these two peaks probably were the highest since 1899. During the same period, the Zumbro River at Theilman reached a discharge of 20, 900 cfs--this figure was slightly less than the highest discharge observed since records began in 1938. In May, the discharge from the melting snow reached the Mississippi River below the St. Croix, and flood conditions existed from Prescott, Wis., to La Crosse, Wis. At La Crosse, the highest stages since 1943 were reached. On June 12 and 13, excessive rains on basins tributary to the Mississippi River between Hastings, Minn., and La Crosse, Wis., resulted in flash floods on the Zumbro, Whitewater, and Root Rivers. Previous maximum flows were exceeded at Theilman, on the Zumbro River, and at Beaver, on the Whitewater River. This report contains records of stage and discharge at 46 gaging stations and records of storage or elevation.at 6 reservoirs. A summary table shows crest stages and discharges at 72 points during the floods of April-June, and similar data are shown for former record floods where it is available. A discussion of concurrent meteorology and references to some outstanding past floods are also included.
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