7371

• UCREFRP Catalog Number: 7371
• Author: Stalnaker, C. B., R. T. Milhous and K. D. Bovee.
• Title: Hydrology and Hydraulics Applied to Fishery Management in Large Rivers.
• USFW Year: 1989.
• USFW - Doc Type: D. P. Dodge, ed. September 14-21, 1986.
• Copyright Material: YES

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hrate an empirical re~re~~IOp. equatJ(ln, One form of thIs
equation follows,
<I) 1'1"1 = V la(Y/D)")
where V,,,) = the nose velocity. V = the mean column
velocity, Y = the nose depth. D = water column depth. a
= regression intercept, and b = regression slope,
Two theoretical approaches can also be sued to translate
the mean column velocity to another point in the vertical.
The first is the one-seventh power law (Schlichting 1968),
which is similar in form to equation (I). except that the
coefficient (a) is set to a value of 1.15 and coefficient (b)
to 0.143:
(2) Vn = V f I.l5(YID)O, 143]
Another approach is to use a form of the Prandtl-von Kar-
man equation:
(3) V(nl = log 33.35 (YID6S)
log 12.27 (D/ D65)
where V(n), V, Y, and D have the same definitions as above.
and D(65) is the particle size diameter that exceeds the
diameter of 65 % of the materials in the substrate.
Coefficients for the Prandtl-von Karman equation were
derived in design channels under relatively uniform flow
conditions (Chow 1959). Habitat studies are usually con-
ducted in stream reaches having nonuniform and complex
channel characteristics that may not conform to the condi-
tions under which the theoretical relationships were devel-
oped. Generally the use of any nose velocity transformation
is superior to the use of mean column velocities in large
streams, but the development of site specific relation is the
best overall approach. Unless further research verifies that
equations (2) and (3) generally hold in large rivers, the
empirical approach exemplified by equation (I) is recom-
mended for determining nose velocities.

Specialized Microhabitats in Large Rivers

Cover - One distinction between assessments of habitat
in large and small rivers is the relative importance of cover
in influencing the tolerances of fish to different hydraulic
conditions. At least two distinct behavior patterns appear to
be associated with cover, and both have relevant implica-
tions with respect to river size. The first behavior has been
described by the use of .. cover-conditional criteria, " which
implies that the ranges of depths and velocities selected by
a species are conditioned by the cover type available (Bovee
1982; 1986). This behavior shift is exemplified by the use
of shallow water when overhead cover is present, but deep
water when it is lacking. The prevailing theory is that, in
the second instance, fish use depth as a form of overhead
cover.
The second category of cover-related behavior is a true
affinity by some species or life stages for certain types of
cover. These species are nearly always associated with
cover, but may tolerate a fairly wide range of hydraulic
characteristics where their favored cover type is present. It
is also conceivable that certain species exhibit a combination
of both categories of cover use.
Such differential behavior can create errors in large-river
analyses, because most of the available information on the
microhabitat requirements of fish has originated from small
streams. The proportion of the surface area affected by
some form of overhead cover is much greater in a small

18

stream than in a large one, If the sre~'le~ has a cover-
condItional hehavior pallern, the depth~ u~cd In a (over-
dominated stream m<ty he much sh<tllower than those used
by the same species in streams without cover, Since most
of the surface area of a large river is devoid of overhead
cover. the suitability of shallow water may. therefore. be
overestimated using criteria from small, cover-dominated
streams.

Edge - A very different effect may occur in species that
do not use depth as cover and are rarely found in the absence
of structural overhead cover. In large rivers, the only area
with an appreciable amount of structural overhead cover
consists of a strip along both edges. What happens in the
middle of the river is of little importance to the habitat needs
of these species, except by its hydraulic connection with the
edge. For these species, an investigator can often confine
most of the detailed habitat description to the edges of the
stream, and measure the center of the channel only superfi-
cially. The center cannot be ignored, however, because the
water level in the main channel controls the depths and
velocities along the edges.

Backwaters - Other specialized microhabitat types,
characterized by low current velocities or specific substrate
types, may also occur only along the margins of the stream.
These areas may compose less than 10 % of the surface area,
but contain 90 % of the fish in a large river. Examples of
these specialized habitat features include side channels
around islands, connected sloughs and oxbows, and the
mouths oftributaries. The investigator is sometimes primar-
ily interested in these localized areas, but more often
includes them in conjunction with other habitats and an anal-
ysis of the entire channel. It is usually possible to treat these
specialized habitat types as separate stream components,
essentially independent of one another. This approach facili-
tates data collection and analysis, but requires additional
data and special procedures for implementation.

Floodplains - Another type of specialized microhabitat
that may be important in large rivers is the floodplain.
Although it might be argued that all streams have flood-
plains, their size and extent appear to increase with stream
size. As the discharge increases in many rivers, the avail-
able suitable habitat tends to decrease due to excessive
velocity in the middle of the channel. This phenomenon is
related more to the channel structure than to the size of the
river, but the general trend is for the usable habitat to be
compressed toward the river margins, as the discharge
increases. Following this logical sequence, it is likely that
the most constraining conditions will occur at or near bank-
full discharge. Once the river overtops its banks, the amount
of usable habitat expands rapidly as the floodplain is
covered. Unfortunately, many riverine habitat studies have
not included the floodplain as part of the model. Conse-
quently, overbank flows are erroneously simulated as
within-channel flows. This error creates several problems
in the habitat analysis, not the least of which is the omission
of the floodplain microhabitat. The relation between stage
and discharge that holds for flows up to bankfull sometimes
changes when the river expands onto the floodplain. Failure
to adjust for this expansion results in the overestimation of