As the largest river basin in the United States, the Missouri River’s critical water resources drive agriculture, industry, and environmental ecosystems from its headwaters in the Rocky Mountains down through Kansas and Missouri. Drought is a substantial problem for the western United States, where water demand has already exceeded water supply in many places. Evidence suggests that recent 21^st century droughts in California and the Colorado River basin have increased in severity, surpassing that of historic droughts. However, this type of understanding of droughts in the Upper Missouri River Basin is lacking. From the year 2000 to 2010, the basin was besieged by a drought more severe than any other in instrumental record. Known as the “turn-of-the-century drought,” this event lacks adequate records needed to better understand potential long-term drought variability in the region, hindering efforts to plan for future drought.

In a new study partially funded by the North Central CASC, researchers, including Alaska CASC scientists Stephen Gray and Jeremy Littell, supplemented drought planning efforts by using tree-rings to develop 17 reconstructions of historic, natural streamflow in the Upper Missouri River Basin. Tree-rings offer much information on historic regional climate conditions. The data collected here provided scientists with 1,200 years of information from which historic streamflow could be reconstructed. This information was then combined with reconstructed regional runoff-season climate information to explore temperature as a driver of drought and place recent drought in a long-term climate context.

Importantly, the results showed that warming temperatures have contributed to increasingly severe droughts, by decreasing runoff efficiency in the Upper Missouri River Basin since the 1980s. Extreme heat and higher evapotranspiration were found to be related to low-flow conditions in the basin, a relationship that has become more prominent over the past 100 years. Snowpack is the primary driver of streamflow in the basin, and recent temperature-driven declines in snowpack and earlier spring snowmelt across the Northern Rockies and West have mirrored drought severity in the Upper Missouri River Basin. Future warming in the basin is anticipated to continue to result in more severe drought events, creating deficits in water resources vital to the region’s human and natural systems. These results suggest that incorporating temperature information into multi-year to decadal forecasts for the basin could help provide a clearer picture of future water supply and support drought planning efforts.

This study is a product of a larger North Central CASC funded project, Characterizing Historic Streamflow to Support Drought Planning in the Upper Missouri River Basin. This research was also highlighted in a recent news article published by The Washington Post.