Wyoming-Montana Water Science Center
Floods of May 1978 in Southeastern Montana and Northeastern Wyoming
Intense rain and some snow fell on previously saturated ground in southeastern Montana and northeastern Wyoming during May 16-19, 1978. The 7.60 inches that fell within a 72-hour period, measured at Lame Deer, Montana, set a record for the month of May in that region.
Widespread flooding occurred in the drainages of the Yellowstone River and its tributaries as well as the Belle Fourche, Cheyenne, and North Platte Rivers. The previous maximum flood of record was exceeded at 48 gaged sites, and the 1-percent-chance flood was equaled or exceeded at 24 sites. Flood damage was extensive, exceeding $33 million. Nineteen counties in the two States were declared major disaster areas.
Mean daily suspended-sediment discharges exceeded previously recorded maximum mean daily values at four sites on the Powder River. The maximum daily suspended-sediment discharge of 2,810,000 tons occurred on May 20 at the Powder River site near Arvada, Wyoming.