25-08-2017, 09:32 PM
INSTREAM FLOW METHODOLOGIES: AN EVALUATION OF THE TENNANT
METHOD FOR HIGHER GRADIENT STREAMS IN THE NATIONAL FOREST
evaluate metodology.pdf (Size: 1.44 MB / Downloads: 98)
ABSTRACT
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.
Background
There is currently a growing conflict in the use of the water resources within the U.S. and
throughout the world (Jackson et al. 2001). In 2001 it was stated that “…over half of
accessible freshwater runoff globally is already appropriated for human use” (Jackson et al.
2001 p.1027) citing (Postel et al. 1996). And it is estimated “…that human appropriation of
accessible runoff could climb to 70% by the year 2025” (Postel 2000 p.941). In the western
United States the flow that is left in the streams has been greatly diminished by diversions
for off stream uses (Gillilan and Brown 1997). “…At least 90% of total water discharge
from U.S. rivers is strongly affected by channel fragmentation from dams, reservoirs,
interbasin diversions, and irrigation…” (Jackson et al. 2001 p.1027). In the western U.S., the
main governing law over water rights is the prior appropriation doctrine that was put into
place over a century ago. The prior appropriation doctrine upholds the right of private
property owner to use water for specified uses, and protects senior water rights holders from
junior water rights holders. Prior to the growing environmental awareness, in the 1960’s and
1970’s, the prior appropriation doctrine precluded obtaining instream flow rights for aquatic
dependent species such as fish. To protect plants and animals that rely on water, there needs
to be a continued shift in the ability to obtain instream flow rights.
Purpose and Objectives
The purpose of this study is to determine if Tennant’s original results (percent of average
annual flow) apply, and under what situations, to streams in the mountainous West (U.S.).
Professional staff for water management agencies can use the results of this study to
determine when the Tennant method is appropriate for use in quantifying the amount of
flow needed in the stream to maintain fish habitat. The three objectives for this study are to
determine if: 1) Tennant’s original width, depth, and velocity characteristics represent
western mountainous streams, 2) Tennant’s fixed percent of average annual flow (AAF)
values are constant across mountain streams, and 3) there are any regional characteristics
that affect the applicability of Tennant’s method.
The first objective is to evaluate Tennant’s original width, depth, and velocity
measurements. The values that Tennant suggested for the average width (percent width),
depth, and velocity for each percent of AAF value will be tested. The range of values
calculated in this study for width, depth, and velocity will then be compared to the values
that Tennant employed using a standard hypothesis test to determine if the mean of the
study population is different from Tennant’s values.