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Feedback Mechanism in a Chaparral Watershed Following 1

Burchard H. Heede2

The Problem watershed and in the channels fol­ tion is 1100 m; bedrock geology con­ lowing wildfire in chaparral, to proj­ sists predominantly of deeply weath­ Natural recovery of fol­ ect future developments, and to dis­ ered coarse-textured Precambrian lowing wildfire in chaparral water­ cuss management implications. granite. In spite of relatively high an­ sheds leads to changes in microto­ nual precipitation (677 mm), the cli­ pography. Because chaparral does mate must be considered semiarid. not regrow uniformly, a mosaic pat­ Past Work Only 33% of the annual precipitation tern results. This regrowth often falls in summer when extremely high forms barriers to delivery Much has been written about temperatures (reaching 43° C) reduce from uphill bare sites (Heede 1988). \\'ildfires in chaparral watersheds, as the effective precipitation substan­ Soils derived from coarse-grained well as the catastrophic conse­ tially. The vegetation cover consists granites, which contain very little quences. But to this writer's knowl­ of chaparral that has regrown since binding materials are highly erodible. edge, none of the studies spanned an intensive wildfire 29 years ago. Thus, relatively large volumes of relatively long time periods-two Today, about 95% of the original sediment are deposited at the uphill decades or more. Thus, immediate canopy has been restored. edge of the vegetation barriers, or fire effects were the focus of these El Oso Creek is an ephemeral buffer strips. Since these mounds and investigations. In contrast, Heede et stream. Following the wildfire, im­ other ground surface undulations al. (1988) reported on sediment deliv­ mense amounts of sediment were remain uncompacted for relatively ery linkages in a chaparral watershed deposited in the channels during long time spans, at least three dec­ covering a period of 26 years. They subsequent intense storms. Although ades in our case, their stability de­ showed that postfire developments the total deposits were estimated at 6 3 pends on the soundness of the buffer are complex and can only be inter­ 2.5x10 m , based on seismographic strips. preted if both the watershed and the investigations,3 it is believed that If an intense storm follows a wild­ channel network are investigated. part of this amount originated from fire, another type of sedimentation The essence of their findings was that earlier fires. In the main channel, fills occurs in the channel network. Be­ the watershed and the channel net­ up to 25 m were found. cause deep, loose and coarse sedi­ work were not in the same geomor­ ment deposits favor subsurface phological stage. While sediment de­ flows, most surface flows are small livery from the watershed to the Methods and immense volumes of stream channels had ceased, sedi­ are deposited in the channels. ment movements continued within Prefire and sequential postfire aer­ The objectives of this paper are to the channel network. ial photographs were used to deter­ identify sediment processes on the mine qualitatively the development 1Poster paper presented at the confer­ ence, Effects of Fire in Management of Study Area 3Laird, J. R.: Harvey, M.D. 1986. Com­ Southwestern Natural Resources (Tucson. plex-response of a drainage basin to geo­ AZ. November 14-17. 1988). El Oso Creek watershed (drainage morphologically-effective fire. Tempe, AZ: 2Research Hydrologist, Rocky Mountain 2 U.S .Department of , Serv­ area, 2.5 km ) is located in central Forest and Range Experiment Station, For­ ice, Rocky Mountain Forest and Range Ex­ estry Sciences Laboratory, Arizona State Arizona on the east flank of the periment Station. 799 p. (unpublished re­ University Campus. Tempe, AZ 85287-1304. Mazatzal Mountains. Average eleva- port).

246 of vegetation and conditions. mined. Latest developments were troughs (fig. 2). Sheet metal strips at Relative chaparral densities could be verified on the ground. each side of the troughs assured this obtained. The loci and extent of ero­ Ten microwatersheds, ranging in catch. Collector tanks made possible sion and deposition as well as their size from 0.01 to 0.2 ha, were selected volumetric determinations of flows changes with time were also deter- to represent a range of vegetation and sediment concentrations. conditions on different lithologies, elevations, and slope angles for the measurement of overland flow and Results sediment delivery (fig. 1). These mi­ crowatersheds also have different Judging from the first postfire ground cover characteristics. The photographs, storms following the term microwatershed was used, be­ intense wildfire must have led to in­ cause they are larger in size than creased overland flows and immense those used in traditional plot studies, volumes of sediment delivered into and the boundaries consisted of the stream network. This postfire be­ natural overland flow divides wher­ havior is demonstrated by our field ever possible. Where divides were research on erosion pavements (table not sufficiently pronounced, artificial 1). Now, nearly three decades after watershed boundaries were created the fire, overland flow from bare using sheet metal strips sunken into sites (erosion pavements) still aver­ ···,;/"'.'j~t the ground surface. ages more than 100 times that from "' ·!·.·.;. Ground cover characteristics were chaparral buffer strips. It must be ',?·:;~~:;.~ represented by erosion pavement (3 assumed that immediately after the -~"f/ffi~._r.t~.f . ~ ·,~,. microwatersheds) and erosion pave­ fire, this difference was considerably --~~~~~-~ ment with a vegetation buffer strip larger. DeBano (1966) has shown Figure 1.-Uphill view of part of an installa­ uphill from the measuring station (3 induce hydrophobic soil tion below a chaparral buffer strip; (A) microwatersheds) (fig. 2). The buffer conditions, which cause non-wetta­ chaparral buffer strip; (8) collector trough strips consisted of relatively dense bility of the soils and increase over­ for overland flow; (C) conveyance pipe; (0) chaparral stands, 2.5 to 4.0 m wide. land flow and sediment transport. supercritical flume; (E) flume for intake to sediment pumping sampler. Pipe in fore­ Overland flow and sediment were On our bare microwatersheds, ground leads to a collector tank. caught by 4-m-long sheet metal sediment delivery was on the aver­ age over 300 times larger than on bare microwatersheds with buffer strips (table 1). Due to the hydropho­ bic soil properties immediately fol­ lowing 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 re­ duced overland flow and caused substantial accumulations of sedi­ ment 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 re­ quired to move the sediment. As the first postfire photographs show, the tributary channels were Figure 2.-Looking across a microwatershed of erosion pavement with chaparral buffer strip clogged by sediment and the main at the downhill border. channel lost depth and widened con-

247 siderably. Apparently, stream com­ Conclusion available sediments, should a wild­ petence was not sufficient to move fire strike the watershed in the fu­ the incoming material through the The localized sediment accumula­ ture. This possible chain of events system; thus deposition occurred. tions behind buffer strips represent constitutes a negative feedback Today, practically all channel banks an enormous reservoir of easily mechanism as follows: are lined by chaparral buffer strips. Even south aspects along El Oso / buffer strip loss ~ Creek developed strips of substantial width (15 to 25m). Greater soil depth sediment depositions massive erosion and higher soil moisture seem to fa­ vor this development. Now that sediment movement "'- regrowth of strips / from the slopes has largely stabi­ lized, relatively clear water reaches the channels. When clear waters en­ ter the clogged tributary channels, the available free water energies be­ gin to move the sediment. Channel scour has started in the headwaters and is proceeding downstream. Flow entering the main channel is ab­ sorbed by the deep and porous sedi­ ment 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 capac­ ity. As more deposition occurs, the storage sites widen, surficial flow depth decreases, and even more sedi­ ment is stored.

Figure 3. -Excavation of the loose sediment deposits uphill from a buffer strip. The de­ posits increase in depth downhill. Approxi­ mate depth of this excavation is 0.40 m. Figure 4.-An in-channel fan of sediment formed by flows from a tributary. Long arrow de­ Retractable tape measure (arrow) in the notes direction of tributary flow. Short arrow shows flow direction in the main channel of El excavation denotes scale. Oso Creek.

248 Thus, discouragingly, the restora­ Heede, B. H.; Harvey, M.D.; Laird, J. tion of a burned chaparral watershed R. 1988. Sediment delivery link­ sets the stage for the next cata­ ages in a chaparral watershed fol­ strophic event, should an unusual lowing a wildfire. Environmental storm follow a fire. Management 12(3): 349-358. During three postfire decades, sediment redepositions in the stream network have not led to channel res­ toration, except in the headwater reaches of the tributaries. Large vol­ umes of sediment are still ready for transport by an exceptional flow. Hence, long-term instability charac­ terizes the main channel.

Management Implications

This research suggests that con­ ducting relatively frequent pre­ scribed fires with low intensities could reduce the rates and time frames at which sediment is deliv­ ered to the channels. This would re­ duce the likelihood that an intense chaparral wildfire would radically alter stream system morphology by the movement of large volumes of sediment. Carefully conducted pre­ scribed fires could in many cases ex­ clude burning of the buffer strips lin­ ing the channels, thus further reduc­ ing 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. De­ partment of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station. 8 p.

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