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Lessons from a Flooded Landscape

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Lessons from a Flooded Landscape "INSIDE ~ Legacy of Knowledge 2 Two Floods Compared 3 Integration 4 Biological Dynamics 5 Roadbuilding & Logging 5 F I N D I N G 5 issue one I february 1998 ccS~~t;het WC0'w~thiA11vt"~"Lewis Thomas LESSONS FROM A FLOODED LANDSCAPE n the soggy morning of February 6, dramatic power to such a huge landscape "This is when the physical work of the 1996, Grant, a fluvialgeomorphol- event, a feel, a smell to it," Grant says. "It landscape gets done," says Grant. "More Oogist at the Corvallis Lab, logged was absolutely the high point of my sediment and debris of every kind, from on to the Internet to check the gauging career to date, the field experience you the boulders and the trees to the finest stations in and around the H.J. Andrews dream about." silt, entered the main streams in those Experimental Forest, about 100 miles 24 hours than will in probably the next Swanson vividly describes the kahwoomp southeast. When he saw that the McKenzie 40 or the previous 30 years." And thus sound of giant boulders rumbling along the River was going up 1,000 cubic feet per more dramatic change was wrought upon streambed, and the "Rip City!" experience second every hour, he knew some serious the landscape than will occur again until the of watching whole old-growth trees, with chaos was at hand. next "big one." root wads intact, racing down river chan- Grant and Fred Swanson, a Forest Service nels. He recalls standing beside a flooded "One thing the '96 flood has really empha- geologist and Andrews Forest project main channel watching the rapid approach, sized," says Swanson, "is the tremendous leader, headed immediately for the moun- trunk first, of an old-growth tree captured value of Forest Service watershed research tain forest. After all, the last major flood by floodwaters. "The tip of the trunk in hanging together through all the decades event, in 1964, had predated both their lodged in the bank right at my feet. Then of boredom. It's a tricky balance between a research careers. the force of the current took the root wad maniacal persistence in collecting baseline and swung it around to lead downstream. data while very little seems to be happen- Both reacted viscerally to the scene of the The current yanked the treetop out of the ing, and yet responding to the current fash- flood, once they were literally standing in it bank, and on it went." ions in science or the latest management- during the next two days. "There is a policy issue." For it is precisely those boring baseline data, decades' worth of them, that let researchers truly "measure" the events of the flood. Their patterns through the decades of boredom provide a context for understanding major flood events. far(,twpv~~bcr~ ~cULt-CtI, ~ wO'fth. of t'he-rn" ~ let' v~chef'"1r tv~"~e:J~EW~ of~~ Fred Swanson Gordon Grant ("Torrents of Change") on mountain streams ATIle /996 f100d crystal1i:ed sciemists' thinking "bolll f100d effects ,,"d dl'llll/llics. 1 EDITOR'S NOTE The PACIFIC NORTHWEST RE- SEARCH STATION sen'es society by improving the understanding, use, and managemellt of natural resources. This monthly publication presellts science findings for people who make and influence decisions about managing land. In ourfirst issue, we describe research that reflects our responsive- ness to natural events and our ability to address issues over time. Floods can bring tragedy in the wake of their destruction. Floods also are a natural A. Floods do the physical work of the landscape. process that has shaped our land- 1 scapes. We,hope that our scientific LEGACY OF KNOWLEDGE BUILDS information helps people make wise FROM THE FLOOD OF '96 choices that influence floods associ- ather than dramatically changing exist- The linkingof all these processes creates what ated with forest lands. ing ideas about flood effects and researchers call a disturbance cascade: some R dynamics, the '96 flood seems to have cascades have a snowball effect and get larger The next issue of Science Findings crystallized many of the hypotheses, according as they go, others act more like an unrolling will examine another aspect of cata- to Swanson. Perhaps its most valuable legacy rug, and dissipate their energy and effects has been to encourage new ways of thinking. quite rapidly. For example, the making of a strophic disturbances. We will debris flow starts with saturated soils that First, the news story in the natural forest is present findings on how anadromous begin to liquefy. Some flows never make it that floods are not just about a lot of water. into the first small channel, hung up by an old- fish are affected by the dynamics of Yes, there's a lot of water, but all the time growth tree, a lack of content, a failure to aquatic ecosystems. that story is developing, uncountable diverse attract a following. The rug is unrolled, the processes are happening in stream channels energy spent. As ourfirst issue of Science Findings, and on hillslopes. Many of them connected. this represents a beginning. We will On the hillslopes there are landslides, debris But ifthey do gather enough mass, they'll start flows, quantities of snow absorbing water or taking out some streamside shrubs and logs, be making improvements in content melting at various rates, and interactions increasing momentum and power on their and layout as we go along and between the stream and the road system. In way to the main channel. By this time, they're welcome comments. We also would the channels, there is rising water, moving big enough to take on large stands of alder, wood, and sediment input ranging in size from shift boulders the size of Volkswagen bugs, like to expand our initial distribution. silt through gravel to boulders. Everywhere, add their weight to the force of the flow If you have ideas about improving the transfer of potential to kinetic energy. through the channel. The snowball effect. this publication or names to add to FOR FURTHER READING our mailing list, please contact: Baker, V.R.; Kochel, R.C.; Patton, P.C. eds. 1988. Flood geomorphology. New York: John Wiley and Cindy Miner Sons. 503 p. Pacific Northwest Research Station Grant, G.E., and Swanson, fJ. 1995. Morphology and processes ofl'alleyfloors in moulltain stream, P.O. Box 3890 western Cascades. Oregon. In: Costa. J.E.; Miller, AJ.: Potter, K.W.: Wilcock, P.R., eds. Natural Portland, Oregon 97208 l/Iltropogenic influences infllll'ial geomorphology: the wolmanl'olume. Geophysical Monograph No. 89, (503) 808-2135 Washington, D.C.: American Geophysical Union, 83-101. Jones. J.A.: Grant. G.E. 1996. Long-term stormflow responses to c/earcutting and roads in small and large basins, western Cascades. Oregon. Water Resources Research. 32:959-974. USDA Swanson. F.J.: Dymess. C.T. 1975. Impact of clearclltting and road construction on soil erosion by iiiiii landslides in the wester/! Cascade Range. Oregon. Geology. 1:393-396. United States Forest Service NOTE: Also see Andrews flood web page at: http://www.fsl.orst.edu/lter/navigafr.htm: click on Table Department of Contents: click on Special Reports: flood 1996. of Agnculture unscathed, The effects of the flood were DISTURBANCE CASCADES not uniform between basins, streams, or even adjacent reaches of the same stream. Are flood effects completely random? No. Consider the flood's perspective, Grant suggests: each flood sees a different land- scape. In their moments of chaos, previous Channel- floods changed the landscape, and in the ValleyFloor decades of boredom, land use and vegeta- Disturbance tion also altered the scene, Processes ;;'f For example, if a reach of stream was Road/Stream gouged down to bedrock by a debris flow Network 30 years ago, it provides less resistance to Interactions subsequent debris flows, which therefore have more available energy to transport both wood and sediment downstream. If accumulations of wood remained along the high flow line of a stream, these "wood levees" acted as buffers for the riparian areas behind them. If a stand of old-growth firs got knocked down last time, the alder that succeeded in its place was less likelyto withstand the force of a debris flow, and more likelyto contribute wood to the gath- ering mass. Thus the effects of one flood leave their footprints for the next flood. Overall, the '96 flood produced a pattem of irregular disturbance, with greatest changes in small channels affected by debris flows, and in reaches of the main stem that were unconstricted by bedrock and there- fore able to accommodate channel migra- tion. Changes in populations of stream organisms reflected the patchiness, with AElIchj700d .'ee.' II dijleremlalld,"'lIl'l'. Where the I 96.J.flood kllocked d01\'11old gr"'l'th. it I'(/\'l'd till' ,ray large changes in some areas and no .fl'r subsl'quellt older slllllds tofall ill IYY6. detectable change in others. With sufficient force, and speeds of 20 to And yet here is one of the apparent para- 30 miles per hour, even remnant stands of doxes of a big flood event: despite all the WRITER'S PROFILE old-growth along the channel cannot always drama and the devastation, the landslides, Sally Duncan is a science communications withstand the impact of such a channelized the debris flows, and the channel alterations, planner and writer specializing in Forest re- mass, The flood is Intractably at work, some parts of the landscape escape almost source issues. She lives in Corvallis, Oregon. COMPARING RESPONSES TO TWO FLOODS and management activities do affect how a flood plays out tlons, on the flood plains and midslope," explains Swanson, "and its across the landscape, in a sense because they intensify natural objective was to move logs efficiently, not to consider landscape L instabilities in the system.
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