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Feedback Mechanism in a
Chaparral Watershed
Following Wildfire 1
Burchard H. Heede2
The Problem
Natural recovery of vegetation following wildfire in chaparral watersheds leads to changes in microtopography. Because chaparral does
not regrow uniformly, a mosaic pattern results. This regrowth often
forms barriers to sediment delivery
from uphill bare sites (Heede 1988).
Soils derived from coarse-grained
granites, which contain very little
binding materials are highly erodible.
Thus, relatively large volumes of
sediment are deposited at the uphill
edge of the vegetation barriers, or
buffer strips. Since these mounds and
other ground surface undulations
remain uncompacted for relatively
long time spans, at least three decades in our case, their stability depends on the soundness of the buffer
strips.
If an intense storm follows a wildfire, another type of sedimentation
occurs in the channel network. Because deep, loose and coarse sediment deposits favor subsurface
flows, most surface flows are small
and immense volumes of sediments
are deposited in the channels.
The objectives of this paper are to
identify sediment processes on the
1
Poster paper presented at the conference, Effects of Fire in Management of
Southwestern Natural Resources (Tucson.
AZ. November 14-17. 1988).
2
Research Hydrologist, Rocky Mountain
Forest and Range Experiment Station, Forestry Sciences Laboratory, Arizona State
University Campus. Tempe, AZ 85287-1304.
watershed and in the channels following wildfire in chaparral, to project future developments, and to discuss management implications.
Past Work
Much has been written about
\\'ildfires in chaparral watersheds, as
well as the catastrophic consequences. But to this writer's knowledge, none of the studies spanned
relatively long time periods-two
decades or more. Thus, immediate
fire effects were the focus of these
investigations. In contrast, Heede et
al. (1988) reported on sediment delivery linkages in a chaparral watershed
covering a period of 26 years. They
showed that postfire developments
are complex and can only be interpreted if both the watershed and the
channel network are investigated.
The essence of their findings was that
the watershed and the channel network were not in the same geomorphological stage. While sediment delivery from the watershed to the
stream channels had ceased, sediment movements continued within
the channel network.
Study Area
El Oso Creek watershed (drainage
area, 2.5 km2 ) is located in central
Arizona on the east flank of the
Mazatzal Mountains. Average eleva246
tion is 1100 m; bedrock geology consists predominantly of deeply weathered coarse-textured Precambrian
granite. In spite of relatively high annual precipitation (677 mm), the climate must be considered semiarid.
Only 33% of the annual precipitation
falls in summer when extremely high
temperatures (reaching 43° C) reduce
the effective precipitation substantially. The vegetation cover consists
of chaparral that has regrown since
an intensive wildfire 29 years ago.
Today, about 95% of the original
plant canopy has been restored.
El Oso Creek is an ephemeral
stream. Following the wildfire, immense amounts of sediment were
deposited in the channels during
subsequent intense storms. Although
the total deposits were estimated at
2.5x106 m 3, based on seismographic
investigations,3 it is believed that
part of this amount originated from
earlier fires. In the main channel, fills
up to 25 m were found.
Methods
Prefire and sequential postfire aerial photographs were used to determine qualitatively the development
3
Laird, J. R.: Harvey, M.D. 1986. Complex-response of a drainage basin to geomorphologically-effective fire. Tempe, AZ:
U.S .Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 799 p. (unpublished report).
of vegetation and erosion conditions.
Relative chaparral densities could be
obtained. The loci and extent of erosion and deposition as well as their
changes with time were also deter-
···,;/"'.'j~t
"'
·!·.·.;.
',?·:;~~:;.~
-~"f/ffi~._r.t~.f
.
~ ·,~,.
--~~~~~-~
Figure 1.-Uphill view of part of an installation below a chaparral buffer strip; (A)
chaparral buffer strip; (8) collector trough
for overland flow; (C) conveyance pipe; (0)
supercritical flume; (E) flume for intake to
sediment pumping sampler. Pipe in foreground leads to a collector tank.
mined. Latest developments were
verified on the ground.
Ten microwatersheds, ranging in
size from 0.01 to 0.2 ha, were selected
to represent a range of vegetation
conditions on different lithologies,
elevations, and slope angles for the
measurement of overland flow and
sediment delivery (fig. 1). These microwatersheds also have different
ground cover characteristics. The
term microwatershed was used, because they are larger in size than
those used in traditional plot studies,
and the boundaries consisted of
natural overland flow divides wherever possible. Where divides were
not sufficiently pronounced, artificial
watershed boundaries were created
using sheet metal strips sunken into
the ground surface.
Ground cover characteristics were
represented by erosion pavement (3
microwatersheds) and erosion pavement with a vegetation buffer strip
uphill from the measuring station (3
microwatersheds) (fig. 2). The buffer
strips consisted of relatively dense
chaparral stands, 2.5 to 4.0 m wide.
Overland flow and sediment were
caught by 4-m-long sheet metal
Figure 2.-Looking across a microwatershed of erosion pavement with chaparral buffer strip
at the downhill border.
247
troughs (fig. 2). Sheet metal strips at
each side of the troughs assured this
catch. Collector tanks made possible
volumetric determinations of flows
and sediment concentrations.
Results
Judging from the first postfire
photographs, storms following the
intense wildfire must have led to increased overland flows and immense
volumes of sediment delivered into
the stream network. This postfire behavior is demonstrated by our field
research on erosion pavements (table
1). Now, nearly three decades after
the fire, overland flow from bare
sites (erosion pavements) still averages more than 100 times that from
chaparral buffer strips. It must be
assumed that immediately after the
fire, this difference was considerably
larger. DeBano (1966) has shown
wildfires induce hydrophobic soil
conditions, which cause non-wettability of the soils and increase overland flow and sediment transport.
On our bare microwatersheds,
sediment delivery was on the average over 300 times larger than on
bare microwatersheds with buffer
strips (table 1). Due to the hydrophobic soil properties immediately following the fire, sediment deliveries
during the early postfire storms also
must have been much larger than
those of today' s bare areas .
. Increased infiltration into soils
under the buffer strips greatly reduced overland flow and caused
substantial accumulations of sediment uphill from the strips. Deposit
depths up to 0.45 m were measured
(fig. 3). With future depth increases,
water withdrawal by the loose,
coarse, granite-derived sediments
also will increase, and increasingly
stronger overland flows will be required to move the sediment.
As the first postfire photographs
show, the tributary channels were
clogged by sediment and the main
channel lost depth and widened con-
siderably. Apparently, stream competence was not sufficient to move
the incoming material through the
system; thus deposition occurred.
Today, practically all channel banks
are lined by chaparral buffer strips.
Even south aspects along El Oso
Creek developed strips of substantial
width (15 to 25m). Greater soil depth
and higher soil moisture seem to favor this development.
Now that sediment movement
from the slopes has largely stabilized, relatively clear water reaches
the channels. When clear waters enter the clogged tributary channels,
the available free water energies begin to move the sediment. Channel
scour has started in the headwaters
and is proceeding downstream. Flow
entering the main channel is absorbed by the deep and porous sediment deposits, and the incoming
sediment is deposited as
in-channel fans (fig. 4). These fans are
still growing due to lack of surficial
flows with sufficient transport capacity. As more deposition occurs, the
storage sites widen, surficial flow
depth decreases, and even more sediment is stored.
Figure 3. -Excavation of the loose sediment
deposits uphill from a buffer strip. The deposits increase in depth downhill. Approximate depth of this excavation is 0.40 m.
Retractable tape measure (arrow) in the
excavation denotes scale.
Conclusion
The localized sediment accumulations behind buffer strips represent
an enormous reservoir of easily
/
available sediments, should a wildfire strike the watershed in the future. This possible chain of events
constitutes a negative feedback
mechanism as follows:
buffer strip loss ~
sediment depositions
massive erosion
" ' - regrowth of strips /
Figure 4.-An in-channel fan of sediment formed by flows from a tributary. Long arrow denotes direction of tributary flow. Short arrow shows flow direction in the main channel of El
Oso Creek.
248
Thus, discouragingly, the restoration of a burned chaparral watershed
sets the stage for the next catastrophic event, should an unusual
storm follow a fire.
During three postfire decades,
sediment redepositions in the stream
network have not led to channel restoration, except in the headwater
reaches of the tributaries. Large volumes of sediment are still ready for
transport by an exceptional flow.
Hence, long-term instability characterizes the main channel.
Heede, B. H.; Harvey, M.D.; Laird, J.
R. 1988. Sediment delivery linkages in a chaparral watershed following a wildfire. Environmental
Management 12(3): 349-358.
Management Implications
This research suggests that conducting relatively frequent prescribed fires with low intensities
could reduce the rates and time
frames at which sediment is delivered to the channels. This would reduce the likelihood that an intense
chaparral wildfire would radically
alter stream system morphology by
the movement of large volumes of
sediment. Carefully conducted prescribed fires could in many cases exclude burning of the buffer strips lining the channels, thus further reducing impacts on the stream. Certainly,
addi tiona! research is needed to
study the consequences of increasing
the frequency of prescribed fire on
the chaparral associated erosional
processes.
Although channel structures could
be considered where downstream
values dictate control of sediment
transport from the watershed, they
would be very expensive.
Literature Cited
DeBano, L. F. 1966. Formation of
non-wettable soils.. .involves heat
transfer mechanism. Res. Note
PSW -132. Berkeley, CA: U.S. Department of Agriculture, Forest
Service, Pacific Southwest Forest
and Range Experiment Station.
8 p.
249