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Interactions Between Streamside Vegetation
and Stream Dynamics 1
Burchard H. Heede
2
Abstract.--Interrelationships between vegetation and
hydrologic processes in riparian ecosystems must be
considered by managers before they attempt to alter these
natural systems. A 5-year experiment demonstrated that logs
that fall across the channel from streamside forests
dissipate flow energy, maintain channel stability, decrease
bedload movement. and increase water quality.
the evolution of plant communities (Reichenbacher
1984). This recognition focused on the current
morphology of streams, such as terraces,
floodplains, and inside and outside banks of
meanders (Irvine and West 1979, White 1979), but
not on the long-term evolutionary processes of
streams and riparian ecosystems and their
interactions. The investigators, trained in the
biological disciplines, did not concern
themselves with the influences of plant
communities on the physical system of streams.
This aspect was pursued by hydrologists. Heede
(1972 a,b) demonstrated that debris from
streamside forests influenced the hydraulic
geometry of two mountain streams by aiding 'stream
processes toward attainment of dynamic
equilibrium. Swanson and Lienkaemper (1978)
found the combination of forest clearcutting and
large debris removal from western Oregon streams
may lead to channel downcutting. Keller and
Swanson (1979) stated that large organic debris
may either cause or prevent channel erosion, thus
influencing channel form and fluvial processes.
INTRODUCTION
We are beginning to understand more about
natural systems, their dependency on other'
systems, and how these systems develop. This is
an important step forward from simply evaluating
static, present-day appearances. Our approaches
must be based on the knowledge that natural
systems are dynamic, and interact with each
other. Long-term trends must be recogni.zed if we
are td evaluate the state of a system. The
trends will demonstrate the brevity of a present
condition, a transitional stage within evolution
of a system, whether it is physical or
biological.
This paper will focus on the interrelationship between vegetation and stream
systems by analyzing stream hydraulics--not only
in terms of water, sediment, and geomorphology,
as classically performed in the past, but also in
terms of vegetation, specifically riparian and
streamside ecosystems. In a 5-year study, I
tested my hypothesis that log steps formed by
downed trees take the place of gravel bars, and
thereby reduce bedload movement. Log steps and
gravel bars represent adjustments toward dynamic
equilibrium. Significantly. the small mountain
stream studied was never touched by management
activity.
Examining present-day processes in the
context of terrestrial evolution, Heede (in
press) explained the interactions between natural
systems. He showed that natural systems evolve
slowly but consistem:ly toward harmony within and
between the systems.
DYNAMIC EQUILIBRIUH WITHIN AND BETHEEN SYSTEMS
PAST \-JORK
Due to the dynamic nature of systems, change
is the rule and steady state does not exist.
Changes can be caused by endogeneous or
exogeneous developments. EndogeneouB factors
originate from within the systems. Examples are:
channel instability caused by "normal" flm.,
discharges that. by erosion over time, expose a
weak geologic formation (soft shales, for
example); and disruptions in a plant community
caused by normal plant succession. On the other
hand, exogeneous factors originate from events
occurring outside the system--earthquakes,
excessive rainfalls, or climate changes.
An aspect of southwestern riparian
ecosystems that has received much recent
attention is the influence of stream dynamics on
1
Paper presented at the Symposium, Riparian
Ecosystems and Their Hanagement: Reconciling
Conflicting Uses, [April 16-18, 1985, Tucson,
Arizoqa] •
~ Burchard H. Heede is r~esearch Hydrologist,
USDA Forest Service, Arizona State University
Campus, Tempe, Arizona.
54
potential gradient of 6.2%, has an oversteepened
profile. Large bed gravel and occasional bedrock
protrusions prevent excessive erosion rates at
higher discharges. Furthermore, trees and large
branches falling across the channel are
eventually incorporated into the hydraulic
geometry by formiug log steps, or small dams
(fig. 1). At the waterfalls over the steps, flow
energies are dissipated. Upstream from the
steps, flow velocities are reduced due to
tailwater formation. Once sediment accumulates
above the log steps, the deposit gradients will
be lower than those of the original bed, also
reducing velocities.
Endogeneous and exogeneous factors both can
initiate adjustment processes directed at
regaining or attaining dynamic aquilibrium, also
callee Quasi-equilibrium. This is not a true
equilibrium condition, but one that allows rapid
movement toward a new equilibrium after a
disturbance. Severe disturbance of the systeLl
may lead to a total loss of equilibrium for a
long time.
Adjustment processes not only aim at dynamic
equilibrium within the system, but also hetween
systems (Heede, 1n press). Thus, if one system
is undergoing drastic changes, another interacting system wil1. be affected and may also be
forced to adjust. This has important implications for lend management, as will be demonstrated in this report.
When log steps fail due to rotting, or wash
out during exceptional events, d large supply of
downed timber is available to take their place,
although not necessarily at the same location.
This was show'n in two streams about 4 km from
West Willow Creek, where windfalls, about t\l~"ce
the number· of failing log steps, were already
suspended above the streambed (Heede, 1975).
THE STUDY STREAH
West Willow Creek is a first-order stream
located in a mixed conifer forest of the Arizona
White Mountains at an elevation of about 2,700 m.
Streamside vegetation consists of mixea conifers,
but annual and perennial herbaceous vegetation
occupies small flood plains, moist banks, and
bars. The stream was classed quasi-ephemeral
(Heede 1976) because it runs almost perennially.
During the last 20 years of record, it had a dry
channel only 2% of the time. ("Dryness" was
defined as a period of more than 6 consecutive
dry days).
Where trees are not available. transverse
gravel bars fom by bedload movement. Thcse span
the channel from bank to bank, and form small
dams (fir,. 2), much like the log steps. Gravel
bars also reduce flow energy and accumulate
sediment:.
Thcre is an inverse relationship between
numbers of log steps and gravel bars CReede
1976). With large timber supplies, fewer gravel
bars form, because more log Eteps become
establi&hed. Furthermore, there is a strong
inverse relationship between the spacing of these
Characteristic of many small streams of high
mountain areas, West Willow CIEcek, with a
Figure 1.--Upstream view of a log step.
55
surrounded by a comparable virgin forest at
similar elevation. It yielded an average of
47 tons per hectare of forest debris larger than
7.5 cm in diameter. This is important for soil
development and maintenance of high infiltration
rates that decrease overland flow and erosion on
channel banks. The sampling, which includcci the
strea~ channel and strips of 4.5 m width adjacent
to the channel banks, was based on established
forest: fuel inventory sampling procedures (Brown
bed structures and the channel gradient; spacing
decreases with increasing gradient. These
relationships demonstrate a c.ynamic process
toward adjustm~nt of the char.[tel slope gradient.
The bed structures transform the potentiaJ
(original) steep gradien~ into a stepped profile
that dis::dpates energy and results in lower flow
velocities.
The streamside forest not only supplies logs
and limbs to channel and banks, but also large
amounts of small organic material. This material
plays an important role ir. soil development on
banks and stable bars, demonstrated by invasion
of he:rbaceous rip&rian communities that also aid
channel stability. In contrast, the locations
withuut plant cover had raw ground surfaces and
exhibited surfac~ erosion.
197~).
Stream profile, locations of the former log
steps, and shapes of channel cross sections, the
latter spaced about 30 m apart, were measured in
field surveys. Five years later, the surveys
were repeated.
RESULTS
METHODS
If my hypothesis was correct that gravel
bars do not form where sufficient amounts of
d.owned timber Circ available (Heede 1ge1), gravel
bars must form if log steps are eliminated. Five
years after log steps were eliminated, 74% of the
steps were replaced by gravel bars. Relatively
fast adjustment processes were at work.
All log steps were removed from West l.Jillow
Creek, amounting to an average of 17.1 steps per
100 m of channel. Logs were removed with great
care to cause as little channel disturbance as
possible. In addition to the steps, all forest
debris was removed from channel, banks. and areas
adjacent to the banks. This debris consisted
mainly of windfalls suspended over the streambed,
limbs, and failed log steps. Debris was removed
periodically to assure a log-free channel. All
debris W<H: tieposited outside the channel and away
from the banks.
One would expect severe chauges to the
channel in the vicinity of the removed steps
because scarps \lere formed by the sediment
deposits behind the former log steps. On the
average, these scarps were 10 to 20 cm above the
b(;d and thus created small waterfalls.
.A.pparently, the small heights of the scarps were
responsible that only 8% of all log removals led
to knickpoints that advanced upst.ream. The
remainder were stabilized by gravel bars or
transformed into smooth gradient transitions.
Gravel burs either took the place of the former
log steps, providing a rock-armored waterfall, or
sedimem. depositions upstream from the newly'
created grav~l bars buried the scarps.
The arrount of debr1s produced by the forest
was indicated by a survey in a nearby stream,
The temporary loss of gradient control by
reI:1oval of the log steps. which made up 61% of
all bed structures (gravel bars and log steps),
led to au average 6.2% increase in channel cross
section. This increase was caused by the
advancing knickpoiuts that undercut the banks,
destroyed t.he bank toes, causing bank sloughing
and destruction of the herbaceous riparian
vegetation.
Because the stream and forest systems were
unmanaged before and during the study (except for
log step removal), the adjustment processes
toward gradient c()lltrol were natural actions free
from human influence.
DISCUSSION
The replacement of most removed log steps by
gravel bars \vithin a period of 5 years support 5
ny hypothesiB that fallen Jogs can prevent bar
formatiolH-> (Reede 1981). The importance of this
lies in the fad that gravel bars are built by
bedload movement. Logs, incorporated into the
Figure 2.--Upstream view of a gravel bar (between
the two arrows).
56
hydraulic geometry, are channel controls that
waintain the local base level given by the
e;levation of the log step crest. Thus excessive
hedload mOVement is prevented. If the log step
(control) rots or is eliminated, the accumulated
se;diment upstream from it may be set in motion.
Due to sorting proce3ses during sediment
transport, larger gravels can create transverse
bars by anchoring the individual rocks to each
other and to the channel side slopes.
CONCLUSION
The experiment demonstrated that even the
removal of dead and dying treeb, much less the
entire forest, bordering small mountau:.. streams
may initiate intense stream acijustment processes.
These processes would continue until a ne~
equilibrium within the stream and between stream
and riparian ecosystem could be established. The
ptimary impact would be increased bedload and
suspended load transports, thereby causing
decreased water quality and deterioration of
riparian ecosystems. Because forest and Btream
systems interact, if we destroy one, the others
will also be thrown out of dynamic. equilibtium.
Since the bed material consists of many
particle sizes, fine materials, held in place by
larger ones, are also set in motion during
bedload transport. The fines go into suspension
and are carried into downstream reaches as
susper..ded load. Be.dload movement therefore not
only impairs the stream reach where this movement
takes place but also, by decreasing water
quality, large segments of the stream system.
In contrast, maintenance of ~treamside
forests in their natural condition, which
includes dead and dying trees, will also maintain
stream stability and healthy riparian ecosystems.
Effecti.ve laud management must recognize this
interdependency.
Because 39% of all structures were gravel
bars, which were not disturbed, some controls
were left in the stream after log step removal.
Possibly, new gravel bars began. to form rather
quickly. Hence, damage to banks and herbaceous
vegetation was not excessive, with a 6.2%-change
in the average channel cross section. However,
bank sloughing not only removed the riparian
vegetation and top soil, but also exposed the
surfaces to raindrop impact and increased
erosion. These procesl:;es proceeded until channel
controls were reestablished by gravel bars, alid
sediment accumulated again, leading to bank toe
protection and reinvasion of riparian vegetation.
LITERATURE CITED
Brown, James K. 1974. Handbook for inventorying
downed woody material. USDA Forest Service
General Technical Report INT-16, 24 p.
Inter~L1ountain Forest and Range Experiment
Station, Ogden, Utah.
Heede, Burchard H. 1972a. Flow and channel
characteristics of two high mountain
streams. USDA Forest Service Research Paper
RM-96 , 12 p. Rocky Mountain Forest and
Range Experiment Station, Fort ColliI1S,
Colo.
Heede, Burchard H. 1972b. Influence of a forest
on the hydraulic geometry of two mountain
streams. Water Resources Bulletin
8 (3): 523-530.
Reede, Burchard H. 1975. Mountain watersheds
and dynamic equilibrium. p. 407-420.
Proceedings ~.Jatershed Symposium, ASCE,
Irrigation and Drainage Division, [Logan,
Uthh, August 11-]3, 1975].
Eeede, Burchard H. 1976. Equilibrium condition
and sediment transport l.n an ephemeral
mountain stream. In: Pydrology and ~~ater
r.esources in Arizona and the Southwebt.
Proceedings 1976 Meetings Arizona Section,
AmericaIL V!8ter Resources Association and
Eydrology S\!ction, Arfzona Acaderuy of
Science [Tucson, Arizon.a, April 29-Nay 1].
6:97-102.
Heed~, Burchard H.
1981. Dynamics of selected
mountaiu streams in the western rnited
States of America. Zeitschrift fUr
Ceomorphologie. N.F. 25(1):17-32.
Beede, Burchard H. (In press). The evolution of
salmonid stream systems. Proceedings, Wild
Trout III, [Yellowstone National Park,
September 24-25, 1984]. Trout Unlimited,
Denver, Colo.
!rv1ne, F. R., and N. E. West. 1979. Riparian
tree species distribution and succt::ssion
along the lower Escalante River, Utah.
Southwest Naturalist 24:331-346.
Keller, Edward A., and Frederick J. Swanson.
1979. Effects of large organic material on
Obviously, gravel of sufficiently large size
(weight) must have been available for the
formation of gravel bars. Otherwise, when log
steps were removed, other adjustments would have
taken place to offset the channel gradient
increase. Adjustment processes can be ranked in
terms of relative time and energy expenditures
required for the attainment of a new dynamic
equilibrium (Heede 1980). Ordered fro~ small to
large energy requirements, the processes involve
changes in: bed form, bed armor, width, pattern
(alignment), and longitudinal profile.
Typically, the study stream, having an
oversteepened profile, originally adjusted its
bed form (gravel bars and incorporation of log
steps). If gravel is not available in sufficient
size and/or volume, not only gravel bars but also
armor It'ay not form. In a.ddition, narrow valley
bottoms may preclude channel widening or
A.lignment changes (meanders). Renee, the onlv
available adj ustment "lOu1d be in longitudinalprofile. Low~ring of the channel gradient would
require bed degradation, the process with the
most energy expenditure.
Channel cutting (degradation) was reported
by Swanson and Lienkaemper (1978) [or streams
draining the sandstone region of the California
Coast Range (little gravel), after removal of
large organic debris. Obvi.ously, channel
degradation has more severe imp~cts on riparian
communities than does bedload movement and
formation of new gravel bars.
57
channel form and fluvial processes. Earth
Surface Processes and Landforms 4:361-380.
Reichenbacher, Frank W. 1984. Ecology and
evolution of southwestern riparian plant
communities. Desert Plants 6(1):15-23.
Swanson, F. J., and G. W. Lienkaemper. 1978.
Physical consequences of large organic
debris in Pacific Northwest streams. USDA
Forest Service General Technical Report
PNW-69 , 12 p. Pacific Northwest Forest and
Range Experimellt Station, Portland, Ore.
White, P. S. 1979. P&ttern, process, and
natural disturbance in vegetation.
Botanical Review 45:229-299.
58