Estimating Monthly and Annual Streamflow Statistics at Ungaged Sites in Idaho

Extreme variability in streamflow across the State of Idaho can complicate decision-making processes for planners and managers who frequently deal with issues regarding fish and wildlife, water rights, and other land and water uses. Information regarding streamflow variability on a monthly and annual basis is needed to assist these planners and managers in making informed decisions. Currently, this information is available only for sites where long-term gaging stations are located. Because of the need for monthly and annual discharge information at ungaged sites in Idaho, the USGS, in cooperation with the U.S. Forest Service, began a study to develop techniques for estimating long-term monthly exceedence (80-, 50-, and 20-percent) and mean annual discharges. This report summarizes the techniques and regional regression equations used to estimate various monthly and annual streamflow characteristics and describes the associated reliability and limitations of the equations.

Note: An error on page 16, Table 4, equation for Q.20 for November, was corrected on June 26, 2003. On May 20, 2004, the drainage area (A) exponent for May at Q.50 in table 4 on page 16 was found to be in error and was changed to 1.02.

View Water-Resources Investigation Report 01-4093 PDF (2.47 MB)