An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

Using the Surface Water Toolbox for Estimating Critical Flow Statistics

Julie Kiang, Supervisory Hydrologist, U.S. Geological Survey
Brian Nickel, Environmental Engineer, Water Protection Division, Region 10, U.S. Environmental Protection Agency
Jenny Molloy, Lead Environmental Protection Specialist, Water Permits Division, U.S. Environmental Protection Agency

A new Surface Water Toolbox was developed by the U.S. Geological Survey and the U.S. Environmental Protection Agency. The Surface Water Toolbox is a user friendly, downloadable tool for water managers to estimate a wide variety of streamflow statistics. This tool can be downloaded from and enables users to:

• Estimate critical flow statistics such as 7Q10 for gaged and un-gaged streams;

• Use a variety of statistic tests to evaluate emerging trends and identify anomalies.

You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.