See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/233526074 Logging and Forest Roads Related to Increased Debris Slides in Southwestern Oregon Article in Journal of Forestry -Washington- · April 1985 CITATIONS READS 74 64 4 authors , including: Robert Ziemer Humboldt State University 84 PUBLICATIONS 1,511 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: CP Photo Finder View project All content following this page was uploaded by Robert Ziemer on 05 June 2014. The user has requested enhancement of the downloaded file.
Logging and Forest Roads Related to Increased JOURNAL OF FORESTRY Debris Slides in Southwestern Oregon Reprinted from the Vol. 83, No. 4 April 1985 Michael P. Amaranthus, Raymond M. Rice, Nicholas R. Barr, and Robert R. Ziemer north-south bands that curve to the northeast. The study ABSTRACT-Debris slides over a 20-year period were in- area consists predominantly of pretertiary sediments and ventoried on 137,500 acres of forested land in the Klamath volcanics that have been folded, faulted, and intruded by Mountains of southwest Oregon. Frequency during the serpentinized masses of ultra-basic and granitoid rocks at study period was about one slide every 4.3 years on each fault contacts between the bands. The area is generally 1,000 acres-an erosion rate of about 1/2 yd 3 per acre per between 2,000 and 5,000 feet in elevation, and is highly year Erosion rates on roads and landings were 100 times dissected with narrow canyons. Annual precipitation, those on undisturbed areas, while erosion on harvested mainly winter rain, ranges from 50 to over 150 inches. The ureas was seven times that of undisturbed areas. Three- Klamath region produces timber, water, and fisheries of quarters of the slides were found on slopes steeper than 70 high value. It supports a complex pattern of natural com- percent and half were on the lower third of slopes. The munities in response to steep climatic gradients, diverse study area was subdivided into nine geomorphological ero- parent materials, and a transitional geographic location sion response units which exhibited profound differences where species common to the Pacific Northwest and Cali- in natural erosion rates and responses to disturbance. The fornia merge. results serve as a guide to appraising slide risk associated with planned timber harvests or mad construction on for- Sites Studied ested slopes. We used data from 24 randomly selected Total Resource Inventory compartments on the Siskiyou National Forest. M any of the most productive forests in the Pacific The sample, about 137,500 acres, included about 14 percent Northwest grow on marginally stable slopes where timber of the total area of the forest. Using a mirrored stereoscope harvests and road construction increase the likelihood of with 8 X magnification, we inspected 1:15,840-scale aerial erosion. Despite attempts to hold erosion below an accept- photos taken in 1956 and 1976, and recorded site variables and the volume (to the nearest 100 yd 3 ) of mass movements able amount, management-related increases-mainly from mass movements-have been reported over the years, be- entering drainage-ways during the 20-year interval. We felt ginning with Bishop and Stevens’s (1964) study in Alaska that such mass movements would produce almost all of the and more recently that by McCashion and Rice (1983) in effects of concern to land managers. northwestern California. Historically, damage from mass Photo measurements were verified by field checking 125 erosion has concerned land managers in the Klamath of the inventoried 699 mass movements. The volumes of Mountains area as well. Increased mass erosion reduces the first 50 tallied were field checked and corrected to the site productivity and water quality, causes loss of fish habi- field measurements. The comparison of volumes revealed tat, and damages roads and bridges. no bias in the photo measurements. Consequently, no ad- This article reports a study of mass erosion on the justment was made to later photo measurements. Siskiyou National Forest in southwestern Oregon. Objec- Mass movements were categorized according to type of tives were to estimate quantitatively the effects of forest movement. (debris slide or slump-earthflow), ownership management activities on the frequency and volume of (private or Forest Service), and site (natural, road, mine, or mass movements, and to collect information concerning harvest area). Debris slides are defined as shallow sliding conditions at landslide sites which might serve as a guide of residuum or colluvium lying upon bedrock or mantle to appraising future risks of landslides. material layers having higher strength and lower permea- bility. Slump-earthflows are a combined process of earth Description of Area movement involving rotational movement of a block of reg- The Klamath Mountains of southwest Oregon lie be- olith along a failure plane with subsequent downslope tween the southern Cascade Mountains and the Pacific transport by flowing or a sliding of a series of blocks. Natu- Ocean. The Klamaths are characterized by steep, rugged ral slides or slumps were those on undisturbed slopes with terrain, heavy vegetation, and unstable slopes. The overall no apparent relationship to management activity Road- structural pattern of the Klamath province consists of related failures were those occurring within the road prism, adjacent to landings, or having an obvious connec- tion with the road, such as culvert outfall or ditchline ob- T HE A U THORS -Michael P. Amaranthus is soil scientist, struction. Harvest area failures were those within clear-cut Siskiyou National Forest, Grants Bass, Oregon. Raymond M. boundaries with no apparent relationship to roads or land- Rice and Robert R. Ziemer are hydrologist and supervisory ings. hydrologist, Pacific Southwest Forest and Range Experiment Debris slides were by far the most prevalent type of Station, Arcata, California. Nicholas R. Barr, who at the time of mass erosion, comprising about 80 percent of the volume the study was geologic technician, Siskiyou National Forest, is and 90 percent of the events tallied. Because debris slides principal geologist, Mt. Emily Mining Company, Brookings, Ore- can be more clearly interpreted from aerial photos than can gon. APRIL 1985 229
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