Jennifer Mann Jennifer Mann, M.S.

Instream Flow Methodologies: A Validation of the Tennant Method for Higher Gradient Streams in the National Forest System Lands in the Western U.S. [working with the Stream Systems Technology Center of the USFS]

M.S. (Watershed Science) 2006 Colorado State University, Fort Collins, CO, USA 80523-1472
B.S. (Environmental Sciences) 2002 Oregon State University, Corvallis, Oregon, 97331

Mann, J.L., 2006. Instream Flow Methodologies: A Validation of the Tennant Method for Higher Gradient Streams in the National Forest System Lands in the Western U.S.. Unpublished M.S. project, Watershed Science, Colorado State University, Fort Collins, Colorado, USA, 92pp + 7appendices.


In 1976 Donald Tennant introduced a method for determining instream flow requirements for fish, known as the 'Montana method', or more commonly the Tennant method. The method uses a percentage of average annual flow (AAF) to determine fish habitat quality. From 58 cross sections from 11 streams in Montana, Nebraska, and Wyoming, Tennant concluded that 10% of AAF is the minimum for short term fish survival, 30% of AAF is considered to be able to sustain fair survival conditions, and 60% of AAF is excellent to outstanding habitat. These quantities are employed internationally, regardless of physical and hydrologic setting, due to the simplicity of using only the average annual hydrograph.

The purpose of the current study was to determine under what conditions Tennant's fixed percent AAF values apply, to specifically evaluate Tennant's original width, depth, and velocity measurements, to evaluate the applicability of Tennant's percent of AAF, as compared to other methods of determining minimum instream flows, and to determine if there are regional characteristics that relate to the applicability of the Tennant method. Tennant's method was tested to see if percent AAF actually can be used as a surrogate for other hydraulic measures, such as width, depth, and velocity. These physical parameters have been used in other studies to quantify instream flow used for fish. The two other methods that were used in the comparisons were the wetted perimeter method and the physical habitat simulation system (PHABSIM). A set of regional characteristics were used to look for region specific patterns. These characteristics including: stream type, state, ecoregion, and hydro-climatic regime. A total of 151 cross sections were analyzed on seventy river segments throughout the western U.S. (California, Colorado, Idaho, Montana, Oregon, Utah, and Washington). The streams were classified as pool-riffle, plane bed, step-pool, and dune-ripple. This study will offer resource managers additional information on the applicability of the Tennant method for determining instream flow needs for the physical, biological, and social setting.

This study concluded that Tennant's original dataset was not representative of streams in the western United States. Data collected from lower gradient streams in Nebraska followed the patterns set forth by Tennant much more closely, and therefore the Tennant method is more applicable in similar low gradient streams (slope less than 1%). In higher gradient streams the use of the Tennant method should be with caution and be restricted to planning stages of instream flow recommendations. Further validation and method adaptation is recommended when using the Tennant method for higher gradient stream types. The Tennant method should be used in instream flow protection scenarios and not in restoration scenarios because of the method's assumption that the current average annual hydrograph represents the optimal fish habitat.

Advisor: Steven Fassnacht
David Merritt (USFS)
Sara Rathburn (Geosciences)

Last update: SRF, 2016-06-15