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Discussion and Conclusions

The effect of silvicultural activities on water quality is often contentiously debated. Forestry operations have been identified as nonpoint sources of pollution to water bodies draining forest land. Silvicultural activities have the potential to increase sedimentation and alter stream channel conditions (National Council for Air and Stream Improvement 1994). Impacts from these activities are site-specific, varying across the South. Effects depend on elevation, slope, and the rate at which vegetation recovers following harvest. However, in general, if BMPs are properly designed and implemented, the adverse effects of forestry activities on hydrologic response, sediment delivery, stream temperature, dissolved oxygen, and concentrations of nutrients and pesticides can be minimized.

One of the objectives of sustainable forest management is to ensure that silvicultural activities are conducted without significant nonpoint-source pollution of streams and coastal areas. This chapter identified the primary and secondary impacts of silvicultural operations. The following specific management measures should be considered by all forest managers as they develop comprehensive forest management plans. The effectiveness of these management measures to mitigate water-quality impacts is discussed exclusively in chapter 22.

Planning of the timber harvest to ensure water-quality protection will minimize nonpoint-source pollution and increase operational efficiency (Golden and others 1984). Streamside management areas of sufficient width and extent are crucial because they can greatly reduce pollutant delivery. Identification and avoidance of high-hazard areas can greatly reduce the risk of landslides and mass erosion. Careful planning of roads and skid trails will reduce the amount of land disturbed by them, thereby reducing erosion and sedimentation (Rothwell 1978). Proper design of drainage systems and stream crossings can prevent system destruction by storms, thereby preventing severe erosion, sedimentation, and channel scouring (Swift 1984b).

Road system planning is a critical part of preharvest planning. Good road location and design can greatly reduce the sources and transport of sediment. Road systems should generally be designed to minimize the number of road miles per acre, the size and number of landings, the number of skid trail miles, and the number of watercourse crossings, especially in sensitive watersheds. Timing operations to take advantage of favorable seasons or conditions and avoiding wet seasons prone to severe erosion or spawning periods for fish reduce impacts to water quality and aquatic organisms (Hynson and others 1982). Drainage problems can be minimized when locating roads by avoiding clay beds, seeps, springs, concave slopes, ravines, draws, and stream bottoms (Rothwell 1978). Stringer and Thompson (2000) attribute the limited use of topographic maps by loggers and silvicultural operators for many impacts to water quality.

Potential water quality and habitat impacts should also be considered when selecting the silvicultural harvest and yarding systems. It may appear to be beneficial to water quality to use uneven-aged silvicultural systems because they disturb less ground and remove less of the canopy than clearcuts. These factors, however, should be weighed against the possible adverse effects of harvesting more acres selectively to yield equivalent timber volumes. Such harvesting may require more miles of roads and more frequent re-entry into timber stands, which can increase sediment generation. Whichever silvicultural system is selected, preharvest planning should address how harvested areas will be regenerated to prevent erosion and potential impact to water bodies.

Cumulative effects to water quality from forest practices are not well documented (Neary and others 1989, Reid 1993, Vowell 2001). They are related to several processes: onsite mass erosion, onsite surface erosion, pollutant transport and routing, and receiving water effects (Sidle 1989). Cumulative effects are influenced by forest management activities, natural ecosystem processes, and the distribution of other land uses. Timber harvesting, road construction, and chemical use may directly affect onsite delivery of nonpoint-source pollutants as well as contribute to existing cumulative impairments of water quality. The most effective road system results from planning to serve an entire basin, rather than arbitrarily constructing individual roads to serve short-term needs (Swift 1985).

On watersheds where cumulative effects are known to be a problem, the potential for additional water-quality impairments should be taken into account during preharvest planning. Information from previously conducted watershed assessments should be considered. These types of assessments, generally conducted by State or Federal agencies, may indicate water-quality impairments in watersheds of concern caused by types of pollutants unrelated to forestry activities. However, if existing assessments attribute a water-quality problem to the types of pollutants potentially generated by the planned forestry activity, then the problem should be considered during the planning process. If additional contributions to this impairment are likely to occur, planned activities may have to be adjusted or additional mitigation measures may have to be implemented. Alterations may include selection of harvest units with low sedimentation risk, such as flat ridges or broad valleys; postponement of harvesting until existing erosion sources are stabilized; and selection of limited harvest areas using existing roads.