The ability to estimate selected streamflow statistics for ungaged sites was added to Montana StreamStats version 3 on November 10, 2016, and to version 4 on December 8., 2016. Regression equations are available for estimating instantaneous peak flows with annual exceedance probabilities of 66.67, 50, 42.9, 20, 10, 4, 2, 1, 0.5 and 0.2 percent. These peak flows have recurrence intervals of 1.5, 2.33, 2, 5, 10, 25, 50, 100, 200, and 500 years, respectively. In addition, regression equations are available for estimation of low-flow frequencies; mean annual and mean monthly streamflows; and the 20-, 50-, and 80-percent durations for annual and monthly duration streamflows for ungaged sites in western Montana.

Regression equations for estimating these statistics are not available for eastern Montana. All equations assume that estimated flows at user-selected sites are unaffected by regulation. The reports below document the available regression equations for Montana, the methods used to develop the equations and to measure the basin characteristics used as explanatory variables in the equations, and the errors associated with the estimates obtained from the equations:

• Sando, Roy, Sando, S.K., McCarthy, P.M., and Dutton, D.M., 2016, Methods for estimating peak-flow frequencies at ungaged sites in Montana based on data through water year 2011: U.S. Geological Survey Scientific Investigations Report 2015–5019–F, 30 p.  
• McCarthy, P.M., Sando, Roy, Sando, S.K., and Dutton, D.M., 2016, Methods for estimating streamflow characteristics at ungaged sites in western Montana based on data through water year 2009: U.S. Geological Survey Scientific Investigations Report 2015–5019–G, 19 p. 

StreamStats for Montana also includes a tool that tests for the presence of upstream dams and computes the percentage of a delineated basin that is affected by dam regulations.