7916

• UCREFRP Catalog Number: 7916
• Author: Stalnaker, C. B.
• Title: Methodologies for Preserving Instream Flows, The Incremental Method, ‎(from a syposium by the Upper Mississippi River Basin Commission, Nov. 14, 1978)‎
• USFW Year: 1978
• USFW - Doc Type: Instream Flow Management State of the Art Proceedings
• Copyright Material: YES

The stream reach simulation recommended by IFG
uses several cross-sectional transects, each of which is
subdivided into 9 to 20 subsections. The computer pro-
gram then treats each subsection as an essentially
separate channel. For any stage (water surface eleva-
tion), the mean depth and velocity of each subsection is
then calculated for all desired discharges
(Main, 1978a).

An area represented by these values of depth and
velocity is calculated by multiplying the width of the
subsection by half the distance to the next transect
upstream and the next downstream. This representa-
tion is illustrated in Figure l.

The stream reach simulation takes the form of a
multi-dimensional matrix showing the surface area of
stream having different combinations of hydraulic
parameters (i.e., depth, velocity, substrate, and cover
when applicable). Table 1 illustrates a depth-veolcity
matrix, although the analysis is not limited to two
dimensions. The outlined numeral in the upper left-
hand corner of the first matrix refers to 195 square
feet per 1000 feet of stream length having a
combination of depths less than 1.0 feet and velocities
less then 0.5 ft./sec. This is the total summation of
areas within the stream reach with that combination
of depth and velocities. These areas are not necessarily
contiguous.

In order to evaluate the magnitude of impacts
caused by changes in stream hydraulics, it is
necessary to develop an information base for each
species or group of species of interest. This
nformation base is in the form of biological response
criteria.

Biological criteria are primarily aimed at those
')arameters affecting fish distribution which are most
irectly related to streamflow and channel
.norphology; namely depth, velocity, temperature,
and substrate. Cover, a habitat parameter of
paramount importance to many species, is also
indirectly related to streamflow. Cover may be
incorporated into an assessment by evaluating the
'lsability of available cover objects in reference to the
flow parameters around them. Species criteria for
'hese parameters are being developed by the Instream
f"low Group (Bovee, 1978).

The expressed assumption is that the distribution
l.I1d abundance of any species is not primarily
,nfluenced by any single parameter of stream flow,
but related by varying degrees to all streamflow
hydraulic parameters. Furthermore, these criteria
are based on the assumption that individuals of a
species tend to select the most favorable conditions in a
stream, but will also use less favorable conditions
with the probability of use decreasing wher~
conditions become less favorable.

Given a sufficient number of observations and
:neasun~meTl~,. it is possible to determine a species'
)"eference within a certain parameter, such as depth.

...... -

It is also possible to calculate the relativeprobability
that the species will utilize some increment of that
parameter which falls outside of its preferred range
(Bovee and Cochnauer, 1977).

Most flow assessment methodologies used to date
address only one, or occasionally two life history stages
(Stalnaker and Arnette, 1976). Frequently, a
particular life history stage, or a certain time period is
singled out as being critical for the continued well
being of a fish population. For example, spawning
success is commonly considered a critical factor in the
maintenance of a fish population, but habitat
evaluations for fry and juvenile fish are almost
universally neglected. However, under the
incremental method, hydraulic preference criteria
are developed for all life stages (Bovee and Cochnauer,
1977).

Composite Use (Electivity)
The composite usability of any combination of
hydraulic conditions encountered in a stream reach
may be determined from the individual preference
curves for each species and life stage.

Figure 2 illustrates normalized preference curves
for adult small mouth bass for depths and velocities.
For a given increment of each parameter a use
electivity weighting is read directly from the curve.
For example, if the normalized suitability for the
depth increment of 3.5 feet is 0.37 and the weighting
value for the velocity increment of 0.5 ft/see is 0.81,
then the composite usability for adult smallmouth
bass for a depth of 3.5 feet and a velocity of 0.5 is 0.30
(0.37 x 0.81). A composite usability is similarly
calculated for all stream reach subsections and then
summed.

Substrate or cover are also incorporated into this
determination of composite weighting following
similar procedures. If the substrate found with the
above combination of depth and velocity had a
preferred use of 0.90, then the composite weighted use
for that combination of depth, velocity and substrate
would be 0.27 (0.37 x 0.81 x 0.90).

Weighted Usable Area

The weighted usable area is defined as the total
surface area having a certain combination of
hydraulic conditions, multiplied by the composite
usability for the combination of conditions. This
calculation is applied to each cell within the multi-
dimensional matrix which in turn is a summation of
all stream reach subsections.

This procedure roughly equates an area of marginal
habitat to an equivalent area of optimal habitat. For
example, if 1000 square feet of surface area had the
aforementioned combination of depth, velocity, and
substrate it would have the approximate habitat value
of 270 square feet of optimum habitat.

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