Trends in the Timing and Volume of Peak Flow Events in the Missouri River Basin

Long-term changes in streamflow may respond to climate change which will impact management practices for water resources and wetland ecosystems. Previous studies of mean annual streamflow in the Missouri River Basin indicate statistically significant decreases in the western part of the basin and increases in the eastern part over the last 50 years. Trends were further explored from 1950-2007 at three streams in the basin that are part of the USGS Hydro-Climatic Data Network (HCDN): Cheyenne River at Edgemont SD, Yellowstone River at Billings MT, and James River at Scotland SD. Patterns were examined for annual peak flow and flows exceeding the 2-year recurrence interval (RI). Changes in the nature and timing of peak flows are of particular geomorphic importance in that they sculpt and control channel form. The Cheyenne River exhibited a marked decrease in the volume of water conveyed by floods since 1980. From 1950 to 1980, 20 peak flow events exceeded the 2-year RI and conveyed a total of 763,000 acre-feet (AF). From 1980 to 2007, there were only 9 such events and they conveyed 134,000 AF. From 1950-70, the annual peak discharge occurred between May and August. Since 1970, timing of annual peak discharge is more variable, occurring from March to October. The Yellowstone River had 18 peak flows exceeding the 2-year RI prior to 1980 and conveyed a total of 16,300,000 AF, and 11 such events since 1980 conveyed 9,160,000 AF. By contrast, the James River showed increasing volumes conveyed by floods since 1980. From 1950 to 1980, the 11 peak flows exceeding the 2-year RI conveyed a total of 4,210,000 AF, and 15 events after 1980 conveyed 12,200,000 AF. Peak flow exceeded the 2-year RI in 8 out of 9 years from 1993 to 2001. The change in frequency of channel shaping flows and volume of runoff will over time change the geomorphic nature of these streams and rivers.