The heavy rains of early Feb 1996, washed more than just new logs into our streams. Here are some reports on what happpened.
* Preliminary Executive Summary
Post-Storm Aerial Reconnaissance of the Middle Oregon Cascades and Middle Coast Range
prepared by William Weaver, Ph.D. Pacific Watershed Associates Geomorphic Studies
* Erosion & Sedimentation Processes
* Wildland Hydrology
* Erosion Control Box 4433
* Arcata, California
Purpose On February 14-15 Pacific Watershed Associates of Arcata, California, conducted an aerial survey of storm damage in the Middle Oregon Cascades and Middle Coast Range. The purpose of the reconnaissance was to provide an immediate post-storm assessment of the nature, magnitude and spatial distribution of watershed erosion and impacts to stream channels which serve as habitat for anadromous salmonids, resident trout and other riparian resources. A second objective was to visually determine the association between watershed variables, land use practices and landslide activity triggered by the storm. A total of 9 hours of aerial reconnaissance was conducted during which I viewed a portion of a number of watersheds and smaller sub-basin areas. This review was not intended to be a complete or thorough inventory of storm-related landsliding and watershed impacts.
Areas reviewed Rivers and watershed areas reviewed in the Cascades included portions of the North Fork Middle Fork Willamette River, the McKenzie River and the Santiam River basin. In the Coast Range, a number of mountainous areas were also reviewed, in cluding portions of the Smith River basin, the Suislaw River, the Alsea River, the Yakina River and several small coastal watersheds which drain directly to the Pacific Ocean between Reedsport and Waldport.
* Watersheds and stream channels in many drainage basins of the both the middle Cascades and the middle Coast Range have been hard hit by the February 1996 storm. Sub-basins which were observed during this reconnaissance to be especially impacted include Quartz Creek and Blue River (tributaries to the McKenzie River), Quartzville Creek (a tributary to the Middle Santiam River), Hadsall and Knowles Creeks (tributaries to the Suislaw River), and Lobster Creek (a tributary to the Alsea River), among others.
* Damage is widespread and highly variable from watershed to watershed and is not limited to the human infrastructure. Many rivers and stream channels have received extensive new deposits of both organic debris and fresh sediment. Some streams have been impacted much more than others. Some new organic debris may be beneficial in the long run, but the heavy sediment deposition insome rivers and streams is likely to affect channel morphology and aquatic habitat for decades.
* The cause of variations in the extent of watershed damage from basin to basin is under investigation. The greatest concentration of landsliding and watershed damage occurred or originated in recently clear cut areas and in areas with logging roads built on steep slopes. However, not all managed areas were heavily impacted.
* It appears that the greatest damage occurred in watersheds with a combination of steep slopes and/or unstable bedrock geology, recent harvesting high road densities (or roads built on steep slopes), and within an altitude range where precipitation intensities were probably the greatest (1500 feet for the Coast Range and 3000 feet for the Cascades). In the absence of recent land management (roads and clear cutting) it appears that similar watershed areas experienced much less severe damage.
The most heavily impacted watersheds in the Cascades ranged in elevation between 2,500 to 3,500 feet Watersheds lower than 2,500 feet and higher than 3,500 feet elevation showed substantially less damage. Higher watershed areas are less managed and probably retained their snow cover during the storm while lower watersheds do not contain as many managed steep, unstable slopes. In the Coast Range, most damage occurred on managed lands near the 1500 foot elevation, level where slopes are steepest and precipitation was probably the greatest.
Landsliding is the most visible erosion process that has been triggered and dramatically accelerated by the storm. Approximately 60 landslides were documented in the Cascades and twice that many in the Coastal watersheds. An estimated total of 500 landslides were observed during the limited reconnaissance.
There were many landslides in the 100 to 1,000 cubic yard size range, a large number in the 1,000 to 5,000 cubic yard size range and a few very large slides estimated to exceed 10,000 cubic yards in volume.
Most landslides delivered large quantities of sediment to stream channels and many traveled as "torrents" or mud flows for hundreds or thousands of feet down hillslopes and stream channels before coming to rest in larger stream beds and river channels. It is estimated that over 80% of the observed landslides delivered sediment directly to stream channels.
The frequency of landslides within recently clear-cut areas and along forest roads was much higher than for comparable watersheds in middle elevation wilderness or unmanaged areas. Increased rates of landsliding was clearly associated with forest land use activities.
* Some watersheds displayed only localized landsliding while others showed widespread landsliding activity. In both areas, landsliding was almost always associated with managed areas (clear cuts and/or roads).
* Several 40 to 50 year old harvest areas in the Coast Range also showed considerable debris torrenting and landsliding in steep stream channels.
* Logging roads in both the Cascades and the Coast Range were common sites for landslides triggered by the storm. Most of these landslides also delivered sediment to stream channels. Many forest roads have been heavily damaged and will require substantial efforts if they are to be rebuilt. Many probably should not be rebuilt, but will still require considerable preventive treatment to stabilize them against future erosion. Roads located on steep slopes and near stream channels were much more prone to landsliding and failure than roads built on moderate or gentle upland terrain.
*In some watersheds, stream channels have been heavily impacted with sediment and logs.
* Debris torrents in steep channels have scoured the stream bed down to bedrock while large log jams and sediment accumulations have been deposited at the mouths of tributaries.
* A number of main tributary streams have experienced considerable sediment deposition as material is washed in from the steeper channel systems.
* Many of the riparian stands of hardwood trees along major streams and rivers have not been uprooted or destroyed. Flood flows were often high enough to move and rearrange large logs and organic debris within the channel system, and to introduce new material (sediment and logs) from the steeper tributaries, but not enough to strip flood plains of their vegetative cover. Wilderness areas and unmanaged slope areas showed comparably little storm damage and impacts.
* The mid-elevation portions of six different wilderness areas and several unmanaged watersheds were also reviewed to compare storm impacts against managed areas. A few recent landslides were observed in steep stream channels in wilderness slopes but the rate of landsliding (landslide frequency) was very low compared to managed areas especially compared to those which had been recently clear cut. Some wilderness areas contained no visible landslides or stream channel damage.
* In the most impacted wilderness basins observed in the coast range less than 10% of the debris torrent tracks were active this storm. Midslope landslides in wilderness areas were not observed. In contrast, in some recently clear cut basins in the nearby Siuslaw River basin, up to 70% of the headwater swales and torrent tracks were activated by this storm, and midslope landslides were common.
Conclusions: Some tentative conclusions can be drawn from these observations:
* Land use (clear cutting and road building) in some areas and sub-basins has a high risk of resulting in landsliding and stream channel damage. Some types of land use activities may be inappropriate in these areas
* Removal of vegetation from steep swales in watersheds where debris torrenting is a common landsliding process will greatly increase the number of such landslides which occur during a large storm.
* Road construction by side casting on steep slopes is hazardous and will result in greatly increased rates of landsliding and stream sedimentation.
* When the next storm occurs (whether or not it is of lesser or greater magnitude), additional landsliding and erosion will occur in these watersheds. Many existing logging roads show signs of pending slope failure and active clearcutting in these watersheds is occurring on terrain shown to be sensitive to increased landsliding.
* Stream channels will continue to be impacted by the erosion and landsliding which was triggered by this storm event as bare soil areas gully, remaining unstable landslide material continues to fail and move down slope, and sediment deposited in headwater streams is re-eroded and moved down into the larger streams and river systems.
* The impacts of coarse sediment introduced into streams and rivers by landsliding and road failures may persist for decades, depending on transport rates.
* The impacts to lower, larger streams and rivers may actually increase over the near term (next several years) as sediment from the headwater areas is moved downstream and deposited in lower gradient reaches.
* Restoration measures for sediment already delivered to the stream system by landslides and road failures will be impractical and not cost-effective
* Many road systems and road segments that have not yet failed can still be pro-actively treated (upgraded or decommissioned) so they do not fail during future storms. This type of treatment will be most effective and cost-effective. In areas that are most at risk (roads on steep slopes, roads built near stream channels, roads built by side casting, and abandoned and unmaintained roads).
* Prevention of landslides from harvested(clear cut) slopes is dependent on the recognition and avoidance of sites with high risk characteristics. For example, clear cutting some watershed areas exhibiting steep headwater swales appears to have dramatically increased landslide activity. Once cut, the slopes are vulnerable for a period of years until they are well vegetated. In sensitive watersheds or where downstream aquatic resources are threatened, clear-cut harvesting on these sensitive slope locations is best avoided.
* Future restoration and prevention work should be prioritized based on the "value" of down slope and downstream aquatic resources. It is likely that working within stream channels in an attempt to restore productive habitat and conditions will be futile until future up slope sediment sources are addressed through control and prevention.
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AERIAL LANDSLIDE SURVEY OF MAPLETON RANGER DISTRICT FOLLOWING RAINSTORM OF FEBRUARY, 1996 February 14, 1996
Association of Forest Service Employees for Environmental Ethics
The Oregon Coast Range, especially the Mapleton Ranger District of the Siuslaw National Forest, is notoriously prone to landsliding during infrequent (1 in 10- to 20-year) heavy rainfall events. Since the mid-1970s, it has been the Forest Service's standard practice to inventory landslides following a major slide-initiating rainfall event. These studies have found that the great majority of slides result from logging roads, clearcut logging, or both. Further, slides from logging roads are generally larger by an order of magnitude or more than slides from clearcut units.
Though the rate of logging and roadbuilding on federal lands in the Coast Range has lessened over the past ten years as a result of court orders to protect streams from sliding and the President's Northwest Forest Plan, a large infrastructure of roads and cut-over hillsides remains. Like a time bomb waiting to go off, these roads and clearcuts are a legacy from the past that, without rehabilitation and obliteration, threaten the future health of salmon streams, water quality, and, in some locations, residential dwellings.
METHODS AND MATERIALS
Two days following the Storm of 1996, AFSEEE sponsored an aerial inventory of landslides triggered by the massive rains in the Mapleton Ranger District area. Following the classification system used in previous inventories, AFSEEE preliminarily assigned each landslide to one of three types -- road-related, clearcut-related (referred to here as "in-unit"), and natural. Some landslides that appear to be in-unit might have been triggered by road drainage that is not apparent from the air. Thus, a more accurate determination of causality awaits a ground-based survey.
The aerial survey was done using a two-person Cessna airplane provided by Lighthawk, the "environmental Air Force." The survey was performed in six transects over the course of 2.5 hours from the north boundary of the Mapleton District to the Umpqua River at the district's south boundary. Weather conditions for the survey were excellent. Photographs of several landslides were taken.
RESULTS AND DISCUSSION
A total of 185 landslides from the February 1996 storm was recorded. Of these, 114 were in-unit slides, 68 were road-related slides, and 3 were natural, in-forest slides. Consistent with earlier inventories, we believe the number of natural slides to be somewhat greater than recorded given the difficulty of seeing such slides, especially small, riparian slides, from the air. However, even if natural slides are several times more prevalent, the vast bulk of sliding is logging-related.
On average, road-related slides appeared substantially larger than in-unit slides, which is also consistent with previous studies. Road-related slides also appeared to cause more damage to streams; several large debris torrents were triggered by road failures.
The reduction in slide frequency in the southern-most portion of the district appears related to the path of the rainstorm. There was no evidence of sliding on several very steep and dissected hillsides that were recently logged between the Smith and Umpqua Rivers -- an area that is very unstable -- suggesting that the path of the rainstorm was north of this location. News reports of the storm's path and damage appear to support this conclusion.
Flight Path Road Slides In-Unit Slides NaturalSUMMARY
1 (E-W) 7 16 2
2 (W-E) 23 49 1
3 (E-W) 8 20 0
4 (W-E) 16 18 0
5 (E-W) 5 10 0
6 (W-E) 9 1 0
Totals 68 114 3
NEW STUDY: NORTHWEST LOGGING ROAD CONSTRUCTION OUTPACES STREETS, HIGHWAYS
By SCOTT SONNER
Associated Press Writer
WASHINGTON (AP) 12/11/95 -- Construction of logging roads in national forests of the Pacific Northwest has more than doubled since 1960, far outpacing new public streets and highways in the region, according to an environmental group.
More than 325,000 miles of logging roads now crisscross public lands in British Columbia and parts of six Northwest states -- enough to circle the planet 13 times, the Northwest Environment Watch said in a report released Monday.
The region has more than the 220,000 miles of public streets and highways, up 25 percent from 35 years ago, according to the Seattle-based research center.
National forest roads have more than tripled in Oregon since 1960 and more than doubled in Idaho and Washington, the report said.
The study warns of environmental damage caused by logging roads, including erosion and sedimentation in streams that harms dwindling salmon populations.
It urges a halt to logging road construction in the Northwestern states and zero-growth in British Columbia. It applauds U.S. Forest Service efforts to remove roads as a central part of watershed restoration in heavily logged over national forests.
``Perhaps its most surprising finding is that roads have surpassed streams as the most dominant feature of the landscape in the region,'' said Alan Durning, the center's executive director.
``Today, outside of Alaska, more of the U.S. Northwest is accessible to four-wheelers than to salmon,'' he said.
The following table compares highway miles and logging road miles, in thousands of miles through 1994. The first column is streets and highways. The second column is logging roads. The third column is total miles. State logging roads in Oregon and Washington account for an additional 12,600 miles of roads on public lands.
Idaho 35.7 33.5 69.2Editors Note: This report can be obtained from Northwest Environment Watch upon request: 1402 Third Ave., Suite 1127, Seattle, WA 98101-2118 (206)447-1880. Their e-mail address is: email@example.com
Oregon 54.2 72.8 127.0
Washington 72.8 22.0 94.8
SE Alaska 1.8 3.6 5.4
W Montana 6.4 23.0 29.4
NW California 9.7 12.5 22.2
U.S. Northwest 180.6 167.4 348.0
British Columbia 39.2 147.8 187.0
Total 219.8 315.2 535.0
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