Defining Landslide Potential Across Large Landscapes

PNW Pacific Northwest Research Station I NSIDE Resolving Bias in the Data: It’s About Scale .......... 2 Looking for Landslides Across Large Areas ........... 3 Landslides Link Headwaters to Larger Streams .... 3 New Information for Policy and Management ........ 4 FINDINGS issue one hundred one / march 2008 “Science affects the way we think together.” Lewis Thomas A LANDSLIDE IS A LANDSLIDE IS A LANDSLIDE… OR IS IT? DEFINING LANDSLIDE POTENTIAL ACROSS LARGE LANDSCAPES cover. Stream complexity is born out of the physical forces of input and removal. Inputs IN S UMMA R Y are often sudden and dramatic—heavy rains force soil, boulders, and trees to give way Not all landslides are created equal. Some in a landslide. These inputs are steadily have the potential to run out to streams and rearranged then swept away by streamflow. others do not. Some are likely to simplify and damage stream habitat, and others can Kelly Burnett, a research fish biologist at be important sources of gravel and large the Pacific Northwest Research Station, wood, fundamental components of habitat and Dan Miller of Earth Systems Institute have been studying salmon and their stream complexity for salmon and other stream habitats. inhabitants. Forest managers want to avoid negative consequences and promote positive Debris flows are common events that characteristics of debris flows. Until recently, can shape stream habitat in mountainous however, approaches to assess whether regions around the world, particularly in the landslides will enter streams over large areas rainy Pacific Northwest. “A landslide, once have been unavailable. in a stream, can create a debris flow—a slurry of mud, rocks, and organic material Scientists at the Pacific Northwest Research that scours sediment and wood along steep Station and their collaborators at Earth headwater streams—and then deposit this Systems Institute developed techniques to downstream in lower gradient fish-bearing estimate densities of landslides over an channels,” explains Burnett. “But, not all entire region and specify confidence in those landslides enter streams and not all debris Debris flows often transport wood and sediment flows travel to streams containing fish.” estimates. Landslide densities are estimated from headwater streams and deposit it lower in as a function of topography and forest cover the watershed, where it can affect fish habitat. Debris flows are bad for fish—but that is from elevation data, forest mapping, and only part of the story. The negative effects landslide inventories. Existing field- and of debris flows can last from tens to hun- air-photo-based landslide inventories are dreds of years, depending on the magnitude “To put your hands in a river used together to capitalize on the strengths of the debris flow relative to the size of the is to feel the chords that and overcome the deficiencies in each. For stream. “Debris flows can be important bind the Earth together.” the first time, source areas for landslides —Barry Lopez sources of gravel, boulders, and large wood, which are fundamental components of can be differentiated between those likely habitat complexity for salmon. Forest man- and unlikely to affect streams. This can help ow do fish see their habitat? agement may alter many characteristics of decisionmakers more effectively regulate Particularly salmon, darting across debris flows, including how frequent they timber harvest and target forest management interplays of light and dark in a H are, how big they are, how far they travel, and restoration activities. Researchers cold mountain stream. Perhaps they can and their composition” Burnett explains. sense that their survival depends on habitat also found that locations with the highest complexity: the cascades over logjams that Scientists have come to understand, in the probabilities of initiating and delivering excavate deep pools, the shallow riffles past few decades, the nuanced role that debris flows to fish-bearing streams can interspersed with overhanging streambanks landslides and debris flows play in fish be managed by concentrating on a small around slow meanders that provide food and habitat development. Not all landslides percentage of the landscape. K E Y FINDIN G S • Debris flows can be important sources of gravel and large wood, which are fundamental components of habitat complexity for salmon and other stream inhabitants. Forest management may alter many characteristics of debris flows, including timing, magnitude, and composition, affecting stream organisms and habitats. • Aerial photo inventories of landslide density have a bias toward detecting larger landslides and landslides in nonforested regions. Field-based inventories do not adequately estimate the variability over large areas. By combining the strengths of the two methods, researchers were able to estimate regionally accurate landslide density. They found landslide densities were lowest in older forests and highest in recently disturbed forests. • Not all landslides are created equal, but approaches to assess regional differences have been unavailable. Researchers have recently developed a method to identify and rank source areas based on the potential to initiate landslides that can travel to and through a stream and thus affect fish habitat through scour or deposition. are created equal, and scientists and land wood from previous stands, it contains There is a longstanding controversy regarding managers are developing ways to resolve mostly sediment, and lacks a key structural the relationship between logging and landsliding. which landslides are likely to influence element for long-term habitat complexity.” New research shows that, after accounting for the patchiness of the process, there are lower landslide streams and which characteristics determine The distinction between landslides on forested densities in older forests than in young forests or whether the longer term net influence of recently harvested forests. versus nonforested slopes is of particular con- debris flows on salmon and habitat is likely cern in areas managed for timber production. to be positive or negative. “For several decades, controversy has swirled Burnett and Miller created a technique to “Large wood delivered by debris flows is a around the relationship between logging and identify characteristics associated with land conspicuous component of stream habitat,” landslide initiation in the Northwest,” explains sliding and then used it to map likely land- says Burnett. “The wood traps sediment and Burnett. “And despite numerous studies, sci- slide initiation sites across large landscapes. controls channel morphology.” Debris flows ence has yet to completely resolve the debate.” Their approach harnesses the advantages of of trees and logs benefit fish habitat in the Adding fuel to the controversy is the fact that traditional field-based estimates and combines long run. In contrast, when a debris flow previous studies have produced conflicting them with aerial photography and computer originates and travels through an unforested results about the role of forest cover on the modeling to understand some of the conflict- area, particularly one with little downed probability of landslides. ing results in previous studies. ResolVing bias in the data: it’S about scale ow could some studies show that Although inventories from field surveys landslides occur more frequently provide accurate measurements and counts Purpose of PNW Science Findings in logged forests, and other studies of landslides in all forest age classes, field H To provide scientific information to people show the opposite? The answer to this riddle surveys can reasonably include only a small is found in the peculiarities of conducting area, unless of course you have an army of who make and influence decisions about managing land. landslide inventories, which typically come surveyors. “Because of landslide patchiness— in one of two forms: aerial photos or field the simple fact that landslides tend to occur PNW Science Findings is published surveys. Both have inherent strengths and in clusters—landslide counts can vary monthly by: weaknesses. considerably from one location to another Pacific Northwest Research Station even in forests of the same age,” says Burnett, USDA Forest Service “Landslide inventories from aerial photos “so, a field survey in a small area may miss P.O. Box 3890 have the advantage of looking across a broad general tendencies between forest age and Portland, Oregon 97208 area relatively inexpensively,” says Burnett. landslide density that are applicable over a “The major downside is that the number of Send new subscriptions and change of address larger region.” information to [email protected] smaller landslides in mature forests tends to be underestimated from air photos because Working in the Oregon Coast Range, Miller Sherri Richardson-Dodge, editor you can’t see smaller landslides through the and Burnett developed a technique that [email protected] canopy. As a result, regional estimates from combines information from field and air Keith Routman, layout air photos may be biased toward [detecting] photo landslide inventories to harness [email protected] more landslides in unforested areas.” strengths in each. Science Findings is online at: http://www.fs.fed.us/pnw/ United States Forest Department Service The site includes Science Update—scientific knowledge for pressing decisions of Agriculture about controversial natural resource and environmental issues. 2 “We estimated

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