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Channel Response of an Ephemeral Stream in Wyoming
to Selected Grazing Treatments1
Ronald E. Siekert, Q.D. Skinner,
2
M.A. Smith, J.L. Dodd, J.D. Rodgers
Studies of the effects of seasonal grazing on ephemeral
stream morphology are summarized. Results indicate that
spring grazing has no significant effect on channel morphology.
Summer and fall grazing is associated with increases in channel
cross-sectional area, with the degree of these impacts varying
with climatic differences. Seasonal grazing can be used as a
management tool for modifying channel morphology to promote
channel stabilization.
INTRODUCTION
annual precipitation of 7.76 inches. Over onehalf of the annual total occurs in the three
month period April through June. Soils in the
area derive from marine shales and are quite
erosive. The principle land use activity in the
area is livestock grazing on BLM allotments.
Domestic livestock overgrazing is one factor
commonly cited as a major contributor to gullying
and arroyo cutting in the semi-arid western United
States (Peterson 1950, Anteves 1952). Increased
erosion and stream bank destabilization from
riparian zone user activities, including widening
and shallowing of channels, is well documented on
perennial streams (Meehan and Platts 1978, Platts
19~1, Gunderson 1968).
However, grazing effects
on ephemeral channels have not been well explored.
A study exclosure is located near the mouth
of the Middle Fork tributary of the 15 Mile
Creek drainage. It is a 600 ac pasture subdivided into five interior cells. Each cell is
approximately 120 ac in extent. Additional
unfenced study reaches are located above the
exclosure on the Middle Fork, and further down
the drainage system on both the main stem and
the section below the confluence of Middle and
South Fork for general drainage monitoring. The
three interior cells of the Middle Fork exclosure are reserved for repeated seasonal grazing
trials. The upstream and downstream cells are a
non-grazed control and a cell reserved for bank
manipulations respectively. Each pasture
contains two study reaches, one centered on a
meander, the other on a straight channel section. All measurements and installations are
focused on these study reaches with the exception of animal behavior observations (Fig. 1).
Using cross-section methodology developed by
Robinson (1982) to determine site specific impacts
by users of riparian zones, the University of
Wyoming Range Management Depart- ment, with the
cooperation and support of the Bureau of Land
Management and the Wyoming State Department of
Environmental Quality, initiated a five year study
in the fall of 1982. Goals of this multifaceted
study include identifying the effects of seasonal
grazing on a riparian area associated with an
ephemeral stream and manage- ment practices to
reclaim riparian habitats and mitigate downstream
sediment related problems.
SITE DESCRIPTION
HETHODS
The study area is located on the ephemeral 15
Mile Creek drainage in north-central Wyoming.
This is a semi-arid region with an average
Seasonal grazing trials were conducted in
May, June, and September of 1983 and 1984 in the
exclosure. Trials consist of the introduction of
thirty (30) cow-calf pairs to the appropriate
cell for a ten day period. Measurements taken in
association with these trials include: channel
morphology, extended cross-section measurements,
cattle behavior observations, and vegetation
utilization.
1
Paper presented at the first North American
conference, University of Arizona, Tucson,
April 16-18, 1985.
2
R. Siekert - Research Associate, Q.D.
Skinner, M.A. Smith, J.D. Rodgers, J.L. Dodd Associate Professors of Range.Hanagement,
University of Wyoming, Laramie, Wyoming.
276
side). Activities recorded include feeding,
resting, and traveling.
Vegetation utilization by cattle is monitored using multiple transects of marked plants.
Fifty individuals each of eleven species sufficiently abundant to contribute to forage
resources are permanently marked with pins in
each grazing cell. Evaluation of utilization is
accomplished by weight estimation of aboveground
biomass by a trained observer immediately prior
to and following the cattle grazing. All weights
are corrected to a dry matter basis, using plants
harvested at the time of observer calibration and
dried. Additional samples of each species are
collected for crude protein analysis. Analysis
of variance and t- tests are used for data
evaluation.
• -study reach
RESULTS
Figure 1.
Comparison of study and control reaches in
1983 and 1984 show no significant change in mean
reach depth in response to season of grazing.
Additionally, no significant difference in mean
depth is noted on any reach between years. In
both 1983 and 1984, no change in channel area
arising from spring grazing was observed. Summer
grazing in 1983 resulted in an overall increase
in channel area as compared to the control. Fall
1983 comparisons are unavail- able. In 1984, the
study reaches in the summer and fall grazing
cells increased in cross- sectional channel area,
but less so than the control reaches in the same
periods (Table 1).
15 Mile Creek study exclosure
seasonal grazing cells.
Channel morphology measurements are taken on
five equally spaced cross-sections encompassing
each study reach. Data is collected on each
cross-section by stringing a leveled line between
permanent end points. The vertical distance from
this reference to the channel is then recorded at
intervals of one-tenth (.10) of the channel top
width. The position and depth of the interim
banks are also recorded at each cross-section.
Measurements are taken prior to and following each
trial in the study and upstream control cell. The
data collected from these cross-sections is used
to develop mean channel depths and channel crosssectional areas. Changes in area are determined
by subtracting the pre- grazing area from the
post-grazing for both the trial reaches and the
control. F-tests are used to determine if the
mean changes occurring in the grazing and control
cells differ at the a= .10 level.
Table 1.
Means and standard deviationf of
cross-sectional area changes (ft 2 )
in treatment and control pastures by
season and year.
TREATMENT
Extended cross-section surveys are taken
concurrently with morphology measurements. These
surveys are run with a level and stadia rod on
lines extending from the cross-section endpoints
to pins situated twenty feet into the floodplain
on either side of the channel. Stadia readings
are taken at one foot intervals to monitor cattle
trampling effects on the near-bank floodplain.
Data treatment is similar to channel morphology
area analysis.
CONTROL
YEAR
SEASON
1983
SPRING
* SUMMER
FALL
1.30
-1.04
-0.09
6.09
2.51
3.17
1. 04
2.47
4.65
3.07
1984
SPRING
* SUMMER
* FALL
1. 31
-1.37
-1.11
4.30
1. 97
4.78
0.08
-3.63
-5.79
3.73
1. 65
5.00
X
s
s
X
1p
.
.
.
re-graz1ng
m1nus
post-graz1ng
area. Negative
(-) values indicate an area increase.
*Treatment and control reaches are significantly
different at the a= .10 level.
Animal behavior observations consist of
locating each animal in the grazing cell at
fifteen minute intervals and recording their
activity, location by habitat type, distance to
shade and water, and the accompanying climatic
conditions. Observations are conducted for six
days, dawn to dusk, during each trial. Habitat
types are classified as upland, floodplain, and
channel (which includes a 10 yard band on either
In contrast, the extended surveys displayed
no significant changes in 1983 or 1984 in any
trial with the exception of the summer in 1983.
Measurements from that period indicate that the
area under the survey line reference increased.
Behavior observations, conducted in conjunction with the 1984 trials, indicated the
277
percentage of animals located on the channel in
the spring was less than one-half of the percentage sighted there in the s~mmer and fall.
Occurrences in the upland remained relatively
constant, declining somewhat in the summer trial
and rebounding in the fall. Cattle occurrence
in the floodplain declined steadily spring to
fall (Table 2).
channel maintained mean depth through all treatments and trials. Increases in channel area
changes resulted from a flattening of channel
slopes or "dishpanning" of the channel. This
conclusion is supported by plots of crosssections.
All significant morphologic effects in the
grazing cells in 1984 corresponded to increased
cattle occupancy of the channel habitat type.
This change in the animals habitat preference
appears to stem from two factors: 1) forage
related factors such as palatability, nutrient
value, and growth form and 2) environmental
factors--specifically the cattle shading up under
cottonwood trees (Populus sargentii) as the daytime temperatures increased.
Utilization of the vegetation species most
closely associated with the channel displayed a
steady increase spring to fall (25.5% to 61.1%).
Floodplain species utilization increased slightly
as the season progressed (33.1% to 39.4%).
Upland vegetation utilization was similar in the
Table 2.
Percentages of total cattle numbers
observed by season and proportion of
habitat type.
CONCLUSION
HABITAT
CHANNEL
TERRACE
UPLAND
~
%
cattle
~
%
cattle
7.6
12.6
39.5
85.4
54.0
1.3
16.5
12.6
34.5
86.1
49.0
2.3
16.0
19.3
26.3
78.4
57.7
%
PASTURE
~
Spring
2.0
Summer
Fall
%
cattle
%
These results suggest that manipulation of
the season of grazing can be used as a management tool in ephemeral stream situations.
Grazing could be used not only as a passive
factor, as in the withholding of grazing to
preserve a systems integrity, but as an active
agent of change. Conceivably, incised streams
with relatively sheer banks could be modified by
selected seasonal grazing pressure. With proper
attention to vegetation utilization and climatic
factors, channel shape could be modified by
controlled grazing to a rounded form. Channel
modifications accomplished in this manner would
require careful timing and management, but the
intermittant character of flow events would
present the opportunity for vegetative
encroachment and stabilization of the new bank
form. This vegetation, properly managed, would
serve in turn as a sediment trap, promoting
channel healing.
%
spring and summer (30.1% and 35.3%), and climbed
sharply in the fall (51.3%). Threshold levels of
utilization considered detrimental were not
exceeded for most species.
DISCUSSION
Relatively intense short-term grazing,
applied in the spring season, appears to have no
adverse effect on channel morphology in the
Middle Fork exclosure. The degree and character
of the changes associated with the seasonal
trials is quite similar over both years. Apparent variations in grazing cell channel response
relative to the control in a given year are due to
the ungrazed control's response to variations in
flow frequency and volume rather than to
differences in the study pasture's response. The
first year, 198~, proved to be a very wet year,
especially in the late summer. Sediment
deposition on banks was high during the summer
trial as evidenced by repeated difficulties in
locating pins marking plants for measurement of
utilization of bank vegetation. Some of these
pins, originally at ground level, were covered
with sediment over one inch deep in the ten-day
trial period. In contrast 1984 was generally
drier with less frequent flows, the exception
being the fall trial which had two separate flow
events in the ten day period.
LITERATURE CITED
Anteves, E. 1952. Arroyo cutting and filling.
J. Geol. 60:375-385.
Gunderson, Donald R. 1968. Floodplain use related to stream morphology and fish populations. J. of Wildl. Mgt. 32(3): 508514.
Meehan, W.P., Platts, W.S. 1978. Livestock
grazing and the aquatic environment. J.
Soil and Water Cons. 33(6): 274-278.
Peterson, H.V. 1950. The problem of gullying
in western valleys pp. 407-434 In: Applied
Sedimentation, John Wiley and Sons, Inc.,
New York, NY.
Platts, W.S. 1981. Sheep and cattle grazing
strategies on riparian stream environments.
pp. 251-270. In Proc. Wildlife-Livestock
Relationships Symposium, 1981. University
of Idaho, Forest, Wildlife, and Range
Experiment Station. Moscow, ID.
Robinson, J.L. 1982. Development and Testing
of a Stream Morphology Evaluation Method
for Measuring User Impact on Riparian
Zones. M.S. Thesis, University of Wyoming,
Laramie, Wyoming.
The nature of the streams morphological
response to grazing did not vary. The stream
278