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VEGETATION REQUIREMENTS FOR FISHERIES HABITATS
William S. Platts
ABSTRACT: This report discusses the importance
of streamside vegetation to each of the four
habitat components that make up the aquatic
environment. The effects of changes in riparian
vegetation on stream temperatures, streambank
stability, stream nutrients, fish cover, and fish
food are discussed. Questions are presented to
help land managers make intelligent decisions
concerning management of riparian vegetation.
streams and their fisheries. The changes in
the productivity and composition of riparian
vegetation caused increases in stream channel
widths, decreases in stream depths, increases
in stream temperature, decreases in fish food
supplies, and in turn a reduction in fish
populations. Once deteriorated, most stream
channels, unlike the riparian vegetation, are
very difficult, if not impossible, to
rehabilitate over the short time period.
INTRODUCTION
This report discusses the importance of
streamside vegetation to each of the four
habitat components that make up the aquatic
environment: the streamside vegetation
(riparian zone), the stream channel, the water
column, and the streambanks. These four
components integrate as a unit to determine the
quality of the aquatic habitat which in turn
determines the productivity of the fishery.
Geologic structure that gives shape to landforms,
and in turn, to streams is measured in millions
of years. Streams of the Intermountain West
acquired much of their present structure during
the late Pleistocene, about one million years
ago. The surrounding soils developed in
thousands of years and most soils are of no more
than Holocene age. Plant associations around
streams, however, can be measured in only tens or
hundreds of years. These plant associations,
especially under man's influence, are continually
being modified, but because they respond to
changes in management practices, the opportunity
exists to convert present associations to more
beneficial types. The response time of these
rehabilitative changes depends on climatic
conditions and soil fertility. Because the
vegetative component of the fishery habitat can
be manipulated quite quickly, it is often less
costly and much easier to obtain immediate
benefits to the fisheries through vegetation
rehabilitation than through channel changes such
as those gained through artificial stream
structures.
VEGETATION FOR STREAMSIDE COVER
The importance of cover to fish is well
documented by the many studies that found
salmonid abundance declining as stream cover
was reduced (Boussu 1954) and increasing as
cover is added (Hunt 1969, 1976; Hanson 1977).
Binns (1979) found that cover was highly
significant in determining fish biomass in
Wyoming streams; as cover increased fish
populations increased. That often narrow
fringe of bordering riparian vegetation is
critical to building and maintaining the stream
structure conducive to a productive aquatic
habitat. This vegetation not only provides
cover but buffers the stream from incoming
sediments and other pollutants.
Salmonids (salmon, steelhead and trout) have been
on the earth in much their present form for the
past million years. During this long
evolutionary period, while the soils and
vegetation surrounding these fish were evolving
in reaction to climatic conditions, fish were
also constantly adapting their life requirements
to meet these changes. They did this quite
successfully until the entrance of European man,
the first animal they faced who was capable of
quickly changing their surrounding landforms,
soils, and vegetation. The transformation of
many riparian-stream habitats from a natural to
artificial state has already occurred over most
of the West. A century of additive landuse
effects has resulted in major impacts on many
Trees, brush, grasses, and forbs each play an
important role in building and maintaining
productive streams. Trees provide shade and
streambank stability because of their large
size and massive root systems (fig. 1). As
trees mature and fall into or across streams,
they not only cause high quality pools and
riffles to form but their large mass helps to
control the grade and stability of the channel.
In many aquatic types if it were not for the
constant entry of large organic debris (trees)
into the stream, the channel would degrade and
soon flow on bedrock (fig. 2). If this were to
result, there would be insufficient spawning
gravel and few high quality rearing pools.
Tree fall is therefore important and often a
must for maintenance of stream stability.
Clearing large debris from streams should only
be done after thorough study.
William S. Platts is Research Fisheries Biologist
at the Intermountain Forest and Range Experiment
Station, USDA Forest Service, Boise, Idaho.
Brush provides cover, which not only protects
the streambank from water erosion, but its low
184
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Figure 1 .--Trees providing shade and streambank
stability .
Figure 3 .--Brush providing s treambank stability
and fish cover .
Figure 4 . --Grasses fo rming vegetative mats are
effective in preventing s treambank
erosion .
VEGETATION FOR STREAMBANK STABILITY
Behnke (1977) states that th e elimination of
streamside vegetation and the caving in of
overhanging streambanks by animals are the
principal factors contributing t o th e decline of
native trout populations in western streams .
There is nothing new in streambanks eroding or
collapsing, as these processes have been going on
since banks were first formed by s uch events as
glaciation, floods, drought , debris flows , and
ice f l ows . This natural s ur face erosion and mass
wasting of streambanks, however , usually occurred
ove r prolonged time and in eq uilibrium with bank
rebuilding processes . In ot her words, as banks
were na turally being eroded there were just as
many banks being built . During the past century
we have upset this state of equilibrium by
altering the banks much faster than they can be
rebuilt.
Figure 2 .--Stream blowout caused by lack of
channel control and stability .
overhanging height adds cover to the water column
which i s used by fish (fig . 3) . Brush , like
trees, build s s tability in streambanks through
its root systems and litter fall . Grasses form
the vegetative mats and sod banks that reduce
surface erosion and mass wasting of st reambanks
(fig . 4) . Streamside vegetation needs high
vigor, density , and importantly, species
diversity, because each of the vegetative types
plays an important r ole in forming and protecting
the aquatic habitat .
185
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; .. ,: .
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Streambanks border in g s mal l streams (stream order
less than 6) prov ide th e habitat edges or niches
nee ded t o maintain high fish populations . Fish
a re often adap t ed to thi s habitat inte r face
because stable, well-vegetated st r eambanks
provide cover, control water velocities and
temp era tu res , and supply terrestri a l foods. The
condi tion of the streambank often governs the
water dept hs and veloci tie s the fis h must live
in.
Streamside vegetation protects streamb anks by
reducing the erosive ener gy of wa ter, by trapping
sediment s to maintain the s treamb ank , and by
prote c ting the streambank from damage by ice
flows, debris f l ows , a nd animal trampling.
Removal of this vegetation exposes the soils to
dire ct erosion f r om rain or surface runoff .
During floods, the high stream velocities not
only transport high amount s of be dload sediment ,
but they also tend t o l ay down the flexible
s tream side vegetation, such as willows and
grasses, into mats th at hug the s treambank.
These ma t s reduce the water veloc ity along the
s treambank face causing sediment s to set t le out
and become part of the barrks. This deposi tion is
u s ually higher on the convex bank forms (usually
f ound on the inside of meanders) than the con cave
bank forms which are the t ypes that are us ua lly
being eroded the fastest. This deposi tion o f
sedimen t s into the vegetative mats con tribut es
fertilizer to the str eambank soils and increases
plant produc tion and vigor. A compact mass of
s treambank vege tation can contribute
subs t antially to the acqu isi tion of sediments
needed t o build and maintain pr oductive
s treambanks .
,.
,,·····
.. ·
Fi gure 5 .--Continuous season- long g raz ing to
the righ t and non-grazing t o the
left of the fe nce in Big Creek ,
Ut ah . Note the dramatic increase in
the quantity and qua lity of the
ripa rian vegetation inside th e
exclosure.
banks take more e ro s ive force from water than
others . l.fhen a stream meanders , the
cen trifuga l fo rce of the water hitting the
concave bank (the outs ide bank of the curve)
increases velocity and in turn f ric tion on the
bank. The direction of the current i s not only
horizontally downstream but also has vertical
upwelling currents . It i s important, then,
tha t a ll concave banks are well vegeta t ed with
deeply roo ted plants .
Streams of the I nt ermoun tain Wes t are u s ually
icebound i n the winter. When winte r " chinooks" or
sprin g thaws arrive , this ice brea ks up and
s tart s t o drift. Furthermore, when the stream is
i ceb ound, and especial ly during periods of heavy
a nchor ice, the s tream of t e n leaves its original
channel and s t arts forming new c hanne l s . Where
streamsid e vegeta tion is insufficient,
there is
n o protective mat and bank e rosion occur s . This
bank e r osion accelera t es under ce rtain grazing
systems, and under high grazing intensities tha t
eliminate the protective mat.
VEGETATION FOR STREAN TEMPERATURE CONTROL
Streamside vegetation shades the stream and
r educes water temperatures (fig . 6) . Solar
r a dia tion acc ounts for about 95 percent of the
hea t input into Intermountain West s treams
during the midday pe riods in mid- summer .
Summer stream temp era tures have probably
increased in Intermountain West streams over
the past century , as streamside vegetation has
been reduced . Thi s could be part of the reason
for a gr a dua l shift from game fish t o less
desirable non-game fish in many of the s treams .
Many non-game fish t olerate higher s tream
temperature s . In the West, streams tha t have
lo s t the ir riparian vege t ation or have had a
change in riparian plant forms (e.g., from
brus h to grass), are often too warm in the
summer and too cold in the winter. Salmonids
are a cold wa ter fish and most stocks cease
growth abov e 68°F (20°C). Temperature changes
can affec t the metabolic rate of fish, c hange
the dissolved oxygen content in th e water, and
influence ha tching success . Water t empe rature
and dissolved oxygen are inversely related . As
wa t er t empera ture in c reases , dissol ved oxygen
concentration in the water decreases .
Temperature s above 68°F (20°C) have been known
Gr azing strategies, s uc h as con tinuou s grazing,
that provide for l ate s umme r or fa ll grazing ,
reduces th e streambank cover a nd exposes the
soi l s direc tly to the ice or high channel flows
(fig. 5). Rest rotation s tra t egies th a t gr aze
early one year, la t e the next, and rest the
third, c an leave thi s vegetative mat in f a ir to
good condit io n on two out of eve ry three year s of
grazing . Nevertheless, the high-intensity
grazing tha t often occ urs under rest rotation can
counter this pot e ntial benefit . The condition of
the vegeta tive mat in the la te fal l grea tly
influences the stability of the s treambank.
When animals graze directly on streambanks , mass
erosion fro m trampl in g , hoof slide, and
s t reambank cave in , causes soil to move directly
into the s tream. When thi s mass movement of soil
occurs, the only way the s treambank can remain in
the equilibrium is to trap enough sediments to
rebuild itself. Because s tr eams meander , some
186
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.... ~.).:\· ..~
sufficient, but gra sses and fo rb s have little
effec t. Claire and Sto r ch (1979) fou nd willow
cover in an ungrazed area within a livestock
exclosure provided 75 percent more shade to the
st r eam than was found in the adjacen t grazed
a rea where willow abundance was reduced .
Herbicide spraying, road construction , logging ,
clearing, and conversions of brush habi t ats to
grass and fo rb s by grazing , have eli mina t ed
vast areas of brushy species along streams in
the Intermountain West .
VEGETATION FOR FISH PRODUCTION
Streamside vege t ation provides habi t at for
terrestrial insects , which are an importan t
part of the fish diet. This vegetation also
provides direct or gani c ma t erial to the s tream
which makes up about SO percent of the stream ' s
nutrient energy supply for the food chain
(Cummins 1974) . Removal of s treamside
vege tation, there fore, can affect the die t of
fish by reducing both the t errestrial and
aq uati c insect production (Cha pma n and Demer y
1963). Because soils in some watersheds ,
especially of granitic parent ma t erial , provide
insufficient nutrients to th e stream , ripar ian
vegetation become s critical in th e production
of fis h food by providing habitat for
t erres trial insects, that fall directly into
the s tream . The s tream detritus formed from
incoming terrestrial plants is a principal
source of food for aquatic invertebrates that
event ually become food for fish (Minshall
196 7) .
Figure 6 .--A well-shaded stream.
t o completely stop fish migration whil e
temperatures above 77° F (25°C) are of t en le tha l
to salmon and trout (Reiser and Bjornn 19 79) .
Streams can also be too cold for successful trout
s urvival. If winter temperatures fall low
enough, anchor ice can form on the bottom of the
stream . Anchor ice forms when the night s are
cold a nd the sky is clear and the c ha nn el can
radiate off its heat directly to the atmosphere.
Streams with little or no vege tative canopy are
very susceptible t o the formation of anchor ice .
Heavy forma tions of anchor ice can produce a
complete fish kill. Anchor ice can also r edu ce
the water interchange in the channel substrate
a nd thus restrict the oxygen supplied t o fis h
eggs in the gravel .
·-
Cover provides shelter and may be the most
frag ile and important single element affecting
a fishe ry. Streamside vegetation closely
over-hanging the water surface or entering th e
wate r provides cover . Young-of-the-year •
salmonid s are dependent on this cover for their
surviva l, and it needs to be maintained .
Unusual high stream temperatures can lead to
disease outbreaks , cessation of feeding, the
s t opping of mig ration s, a nd inhibition of fish
gr owth. Fish have evolved to s urviv e under the
natural temperature regime in their home streams
a nd whe n man modifies these ranges, the results
can be devast a ting to the fish population.
DISCUSSION
Now that the vegetation requirements for the
aquatic habitat and its f i sheries a r e be tt er
understood, land managers need to find better
answers to c riti ca l ques t ions :
Riparian vegetation not only intercepts a nd
reduces the in ten si t y of so l a r radiation but in
so doing also provides cover in the form of
shade , especially along the margins of the
stream . This type of cove r can be critical to
good fish survival because shaded s treamside
areas are a preferred habitat of juvenile
salmonids .
Certain types of ve ge t at ion are needed to control
s tream temperatu r es . Grasses can provide
overhanging cover but thei r shortness makes them
ineffective in inte rcept i ng the s un ' s r ays ,
except in v ery small streams (stream ord er 1 and
2) . The la r ger the s tream , the higher the
s treamside ve ge t a tion needs to be t o effec t ively
intercept t he s un' s rays. In large streams
( s tream order 6 or larger ), trees must border the
s tream to provide effec tive shadowing. In small
to medium streams (stream order 3 to 5 ) brush is
187
1.
Is my management program providing
for high- quality streamside
vegetation?
2.
How far removed from the natural
state are the riparian areas in my
district?
3.
What are the first indicators that
the streamside vegetation is
increasing or decrea s ing in quality
and how do we measure these
indicators?
4.
How much and what type of vegetation
is needed for streambank stabil i t y
and to develop the canopy needed t o
control stream t emperat u res?
··..·
...
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5.
What vegetation types provide the best
cover? The most fish food?
6.
What methods are available to
rehabilitate degraded riparian
habitats and how long does it take?
7.
What intensity and system of grazing
should I use to protect riparian
habitats and insure their
productivity?
Hunt, R. L. Effects of habitat alteration on
production, standing crops and yield of brook
trout in Lawrence Creek, Wisconsin. In:
Northcote, T.G., ed., Symposium on Salmon and
Trout in Streams. H. R. MacMillan Lecture in
Fisheries: proceedings. 1968 February 22-24;
Vancouver, B.C. Vancouver, B.C.: University
of British Columbia; 1969: 281-312.
Hunt, R. L. A long term evaluation of trout
habitat development and its relation to
improving management-related research. Trans.
Amer. Fisheries Soc. 105(3): 361-365; 1976.
Managers do not ask the first question often
enough. Actually, fisheries specialists should
answer the question and when appropriate
suggest changes in management.
Minshall, G. W. Role of allochthonous detritus
in the trophic structure of woodland
springbrook community. Ecology 48(1);
139-149; 1967.
It took many years for streamside environments
in the Intermountain West to reach their
present altered conditions. It would be
erroneous to expect these environments and
their streams to be quickly rehabilitated. It
has been well demonstrated, however, that the
streamside vegetation component of the stream
habitat does respond much quicker than the
other components when better management
practices are applied (Platts 1981). This
response, in turn, speeds up the rehabilitation
of other stream components, thus giving the
land manager a tool to work with in developing
better streams.
Platts, W. S. Sheep and cattle grazing
strategies on riparian-stream environments.
Proceedings of the Wildlife-Livestock
Relationships symposium; 1981 April; Coeur
d'Alene, ID: University of Idaho, Wildlife
and Range Experiment Station, 1981: 251-270.
Reiser, D. W.; Bjornn, T. C. Habitat
requirements of anadromous salmonids. Gen.
Tech. Rep. PNW-96. Portland, OR: U.S.
Department of Agriculture, Forest Service,
Pacific Northwest Forest and Range Experiment
Station; 1979. 54 p.
PUBLICATIONS CITED
Behnke, R. J. Fish faunal changes associated
with land-use and water development. Great
Plains-Rocky Mountain.Geologic J. 6(2):
133-136; 1977:
Binns, N. A. A habitat quality index for
Wyoming trout streams. Fishery Research Report
Monograph Series, No. 2. Cheyenne, WY: Wyoming
Game and Fish Department; 1979. 75 p.
Boussu, M. F. Relationship between trout
populations and cover on a small stream.
J. Wildl. Manage. 18: 227-239; 1954.
Claire, E.; Storch, R. Streamside management
and livestock grazing: an objective look at the
situation. In: Menke, John, ed. Livestock
Interactions with Wildlife, Fish and their
Environments symposium; Sparks, Nevada 1977.
On file University of California, Davis,
Department of Range Management.
Cummins, K. W. Structure and function of
stream ecosystems. Biol. Sci. 24(11): 631-641;
1974.
Chapman, D. W.; Demory, R. L. Seasonal change
in the food ingested by aquatic larvae and
nymphs in two Oregon streams. Ecology 44:
140-146; 1963.
Hanson, D. L. Habitat selection and spatial
interaction in allopatric and sympatric
populations of cutthroat and steelhead trout.
Moscow, ID: University of Idaho; 1977. 66 p.
Ph.D. Thesis.
188