Water-surface profile and flood boundaries for the computed 100-year flood, Big Muddy Creek, Fort Peck Indian Reservation and adjacent area, Montana

Hydrologic and hydraulic evaluations of Big Muddy Creek were made to determine the magnitude of the 100-year flood and the extent of flooding that would occur as the result of this flood. The magnitude of the 100-year flood was determined to range from 13,600 to 20,400 ft3/s, depending on location. Field surveys were made at 39 cross sections along a 41-mile reach of Big Muddy Creek. An additional two cross sections along the same reach were synthesized. Data from the surveys were used to calculate the water-surface elevation at each cross section using a computer program (WSPRO) developed by the U.S. Geological Survey. The water-surface profile of the computed 100-year flood elevations was then drawn. The profile also shows the streambed elevation and the location of the bridges and cross sections. The computed 100-year flood elevation at each cross section was used to delineate the width of the flood plain at that section. Flood boundaries between cross sections were interpolated using contour lines on topographic maps.