« PreviousContinue »
Rates of timber harvesting accelerated on the Forest in the 1950s. Despite efforts to mitigate potentially adverse effects, logging and associated roadbuilding tended to destabilize watersheds (Ketcheson and Froehlich 1977; Swanson and Swanson 1977; Greswell et al. 1979; Barnett 1982; Bennett 1982). Removal of trees from many unstable slopes destroyed root systems, often resulting in slope failures adjacent to and in the headwalls of streams (Burroughs and Thomas 1977). Some road building resulted in undercutting or oversteepening of unstable slopes and increased landslides. Fish migration routes were blocked by impassable culverts at a few stream crossings, and the increased sediment to streams from surface erosion sometimes filled pools and smothered spawning gravel. Removal of debris from in and along stream channels often degraded the fish habitat by removing logs essential to fish rearing pools. In some cases, excessively "hot" slash burning resulted in soil damage such as surface erosion and reduction of nutrients (Gillmor 1969; Bennett 1982; Barnett 1984).
Management practices have changed in response to current conditions and knowledge. By the end of the 1970s, the Forest was routinely using the following practices:
• Leaving vegetation intact on unstable slopes;
The effectiveness of these practices has not been fully established. As ordered by the U.S. District Court in the Amended Judgement of August 6, 1984, for National Wildlife v. Block, research is under way on the effectiveness of retaining vegetation on unstable slopes (Swanson et al. 1985). Initial findings indicate that identification of unstable slopes and the retention of the desired vegetation around them have been successful in more than 90% of the cases. Ultimate success in preventing acceleration of landslides is still in question.
The failure rate on roads constructed since 1975 is negligible. Fewer than 10 landslides have resulted from roads constructed since 1975 on the Mapleton Ranger District (personal communication with Duane Dipert, former soil scientist on the District). Similar decreases in road associated landslides have been noted on the other Ranger Districts. Proof of the effectiveness of these practices will be demonstrated by improved fish habitat conditions (see discussion in Appendix B for more information).
More recently the Forest has been installing various stabilizing structures in the streams to improve fish habitat; managing riparian areas to improve fish and wildlife habitat, and restoring steep road fill slopes to a stable condition.
The effect of these practices on watersheds is illustrated in Figure III-23, which was developed using a sediment model developed on the Forest. The model was based on landslide occurrence from the mid-1970s through the early 1980s, and on a surface erosion study done during the late 1970s (Bush 1982, Bennett 1982).
Timber harvesting on land of other owners began earlier than on National Forest land. It was generally conducted at a faster rate. Consequences of current harvesting on other lands is assumed to be similar to that which occurred following harvest on the Forest in the 1960s.
There has been improvement in the practices on lands of other owners. Bureau of Land Management watershed practices are similar to those of the Forest Service. The State of Oregon has a Forest Practices Act administered by the Oregon Department of Forestry which has gradually changed the practices on private land. Improvement of road construction on steep slopes was given increased emphasis in 1983. However, only minimal streamside buffers are left along major perennial streams and are generally not left at all on small perennial streams or on intermittent streams. Impacts to fish habitat from timber harvest on private lands are now lower than they were in the 1960s, but much higher than current impacts on National Forest lands.
Watershed conditions should remain similar to current or show some improvement as a result of changes in management practices since the 1970s. Inventories of unstable sites should be more accurate and damage from past practices should recover naturally. Loss of large organic debris in the perennial streams logged prior to the late 1970's will continue to reduce productive potential of those streams for several decades.
Watershed and fish habitat conditions are inextricably linked. From the ridge top to the estuary, natural forces are at work rearranging the rocks, slopes, soils, stream courses, and fish and animal communities. The purpose of watershed management is to allow natural processes, as much as possible, to shape and control stream systems. Such management can have pronounced effects on Forest outputs.
Influencing the amount, timing, and location of changes in natural processes requires recognizing when and where to alter management activities to produce the highest level of a desired effect, or the lowest level of an undesirable effect. Altering vegetation, road construction, applying fertilizers, and burning of organic matter are activities most closely linked with changes in watersheds and fish habitat.
Altering the Vegetation
Killing Vegetation - This includes harvesting trees, burning brush or trees, and applying herbicides, if available (see discussion on herbicides in Chapter I). Harvesting trees, burning logging slash and brush, or applying herbicides to reduce the amount of brush and undesirable trees that compete with planted species for growing sites results in the destruction of root systems. When the roots are destroyed on very steep slopes that are prone to periodic high ground water levels, the strength of the soil alone is insufficient to deter landslides. Subsequent acceleration of landslide rates increases sediment in streams, blockages to anadromous fish travel, and channel scour. High landslide rates generally lower water quality and the diversity, quantity, and quality of fish habitat.
Changing Tree Stand Composition - Changing Forest stands from conifer to alder adds nitrogen to the soil and improves productivity. Changes in species composition, or total organic matter may significantly alter productivity after one or more harvest rotations. Changes in tree species composition and logging also affect the amount of large organic debris available for channel structure and fish habitat. Large conifer logs and root wads must be continuously introduced to stream channels to control rapid changes in the shape and behavior of steep stream systems. As more conifers are removed from streamside areas, the channels become more unstable and fish habitat becomes less diverse.
Altering of Surface and Ground Water - Cutting into, and filling over natural slopes during road construction can increase surface erosion and frequency of landslides. The degree varies depending on location, design and maintenance of the roads. Highly unstable areas are easily destabilized by changes in groundwater.
Altering Slopes - Cutting into and filling over the natural slope disturbs the balance of internal soil and rock strength and can lead to failure of the road and the slopes above and below it. Failures of roads and slopes accelerate the natural landslide rate. This increases sediment in streams, and channel scour; it generally lowers water quality and the diversity, quantity, and quality of fish habitat.
Increasing Growth of Vegetation - Addition of nitrogen fertilizer to the soil benefits vegetation by boosting the nutrient most limiting plant growth in the Coast Ranges. The addition of nitrogen fertilizer to timbered areas increases timber volume. It improves the health and vigor of the vegetation and it also promotes stronger root systems on unstable sites, and rapid regrowth where vegetation has been killed.
Reducing Water Quality - Aerial application of chemicals could contaminate stream systems. Water contaminated by fertilizers encourages the growth of algae, and other types of aquatic plants, that may upset the food chain. There is also the danger of the chemical fertilizer entering domestic and municipal water supplies.