In area coverage and magnitude of peak discharge the floods of April-May 1950 in the Missouri River Basin in North and South Dakota were unprecedented in the area. These floods were characterized by an extremely late spring breakup of ice, by great flood peaks resulting from snow melt, and by two separate floods in the James River Valley in less than a month.

The primary cause of the floods was the rapid melting of the season's great accumulation of snow, one of the deepest on record. In the period between the normal spring breakup time and the actual breakup of river ice, considerably more snow accumulated. Some of this was melted by a few warm days and the melt was stored as water behind snow barriers in upland watercourses. A sudden increase in temperature beginning April 13 and lasting until most of the snow had been converted into runoff resulted in rapid rise of flood waters.

Tributary flood waters made the Missouri River from Mobridge to Yankton, S. Dak., rise to near the maximum recorded discharge. At Sioux City, Iowa, the 1950 flood peak-discharge exceeded any previously recorded by the Geological Survey.

The center of the flooded area west of the Missouri River lay in the Cannonball River Basin which had the greatest water content of snow on the ground just before the ice broke up Floods north and south of this area were relatively less intense. Scattered records of the Cannonball River and a study of newspaper accounts and other information show that the flood of 1950 was greatest since the area was settled. Flooding of the James River at Jamestown was the greatest since 1897, and the floods of April and May 1950 were of nearly the same stage.

Itemized flood damages were made by Federal and State agencies, and relief was sent to the area by the Department of the Army and the American National Red Cross.

Data include records of stage and discharge at 54 gaging stations for the period of flood, a summary of peak discharges and comparative data for past and present maxima, a table of crest stages, and weather associated with the 1950 flood.