How low is the streamflow? Using historical streamflow data to give perspective on current conditions (July 19-25, 2021)
Near-record low streamflow in Little Snake River near Dixon, WY
Many streamgages in Wyoming and Montana are recording streamflow that is very low for the date. The primary reason for these low streamflow is the current drought in both States.
The U.S. Drought Monitor describes the presence and severity of drought. The map describing conditions on July 27, 2021, indicates that nearly all of Wyoming and Montana are experiencing some level of drought; some areas are classified as exceptional.
Many factors are considered when determining drought conditions, such as precipitation and soil moisture, and the manifestation of drought is often lower streamflow.
When comparing streamflow across time periods, it is common to use a 7-day average streamflow to describe the streamflow in order to “smooth out” brief changes caused by quickly dissipating high or low streamflows during the time period.
At two selected streamgages located on the Yellowstone and Missouri Rivers in Montana where on July 27, 2021, the drought was classified as severe to exceptional have noticeably lower streamflow. The streamflow record at these two sites have more than 87 years of data, and currently (July 19-25), the 7-day average streamflow is less than 40 percent of the average for the same time period. The current 7-day average streamflow at the Missouri River at Totson is the lowest recorded 7-day average streamflow during the past 20 years.
Current, real-time streamflow and statistics, such as 7-day average, for these four sites and other Montana streamgages can be found online.
Selected streamgages in Wyoming currently have 7-day average streamflow that is much lower than average for the same time period (July 19-25). The Powder River at Arvada, where most of the drainage basin is classified as severe to extreme according to Drough Monitor, currently has a 7-day average streamflow that is less than 10 percent of the 90-year average for the same time period (July 19-25). The current 7-day average streamflow at all selected sites is low, however streamflow is not the lowest recorded in either the last 20 years or during the entire period of record.
Current, real-time streamflow and statistics, for these four sites and other Wyoming streamgages can be found online.
While the eight selected streamgages describe conditions in major rivers across the two States, they demonstrate that the 7-day average streamflow in relation to past 7-day average streamflow is different at each site across this large region. This variability is similar to the variability of range of conditions shown on the Drought Monitor.