| Water quality encompasses the physical, chemical, and biological character of water. Common parameters used to assess water quality include: suspended sediment, turbidity, dissolved oxygen, water temperature, and key nutrients. Forest watersheds often yield high-quality water because of erosion protection provided by forest vegetation, the forest floor, and forest soils. However, forests are subject to human and natural disturbances that can cause major changes in water quality. Best Management Practices (BMPs) have been developed to reduce impacts from forest management activities. In the following sections key physical, chemical, and biological measures as well as management impacts and variations in water quality will be discussed.
Physical Water Quality Parameters
Key physical water quality parameters are suspended sediment, total suspended solids (TSS), and stream temperature. Most commonly stream particulates are measured as either suspended sediment or TSS. These parameters require collecting a representative sample of the water column to measure solids in suspension. The difference between suspended sediment and TSS is that the former is only the inorganic fraction while the latter also includes fine organic detritus, algae, and other organic materials. In eutrophic conditions (high rates of biological growth) organics can be an important component of TSS. Grab samples can be used to measure TSS, but forest hydrologists now recognize that concentrations vary with changes in stream discharge (flow) so samples are often collected with automatic stage-activated samplers. Forest watersheds tend to be sediment–limited, whereby the stream is capable of transporting more sediment than is readily available. The first storm of a given flow magnitude each year usually produces more suspended sediment than subsequent storm flows of the same magnitude. Similarly, suspended sediment concentrations are highest on the rising limb (when discharge is increasing) of the hydrograph (discharge versus time) than for the same discharge on the falling limb (see Beschta 1987).
Turbidity, which measures light transmission through water, is another commonly measured parameter. Methods and reference standards for turbidity measurements have been rapidly changing in recent years, especially with development of continuous turbidimeters that can be deployed in the field. Like suspended solids, turbidity in forest streams can vary dramatically over a year and tends to be highest during peakflow events. In some regions, high turbidity may also result when algae populations become extreme during low–runoff, warm-temperature periods, especially in eutrophic lakes.
Water temperature is important because it regulates chemical and metabolic rates, especially for “so called” cold-blooded organisms (poikilotherms) such as fish and macroinvertebrates (see biological water quality parameters). Cool water has a higher solubility for oxygen than warm water, which can influence respiration by aerobic organisms. Maintaining shade to avoid increases in direct solar radiation on a stream is one of the principal management practices (BMPs) to protect water quality near forest operations (Brown 1980).
Chemical Water Quality Parameters
Chemical measures commonly of concern for forest streams are nitrogen (N) and phosphorus (P) concentrations. In some regions, pH, acidity, and mercury and heavy metal concentrations may occasionally be of concern, as well as herbicides and other silvicultural chemicals.
Nitrogen and P are two elements that are essential for plant and animal growth (nutrients). At some level these nutrients contribute to healthy streams. At higher concentrations and with sufficient light and time, these nutrients can cause degradation of water quality. The most common concern with nutrients is eutrophication of streams and lakes, the process by which a body of water is subjected to excessive growth of plants. This can lead to fluctuating dissolved oxygen (DO) concentrations, increased turbidity, bad taste, and other impairments to water.
Both N and P come in many forms. The combined forms of N are referred to as Total N. The most mobile form of N is nitrate. Other forms include ammonia/ammonium and various dissolved and particulate organic forms. The combined forms of P are referred to as Total P. Soluble forms include orthophosphate or soluble reactive P and less biologically available dissolved complex organic P. Particulate forms of P can be either organic or inorganic. Either N or P can limit growth in a stream or lake. One dramatic example is Lake Tahoe in California which was historically N limited. Deposition of atmospheric N from industrial northern California has shifted the lake to P limited (Goldman et al. 1993) as a result of atmospheric deposition.
Concentration of hydrogen ions (more precisely hydrogen ion activity) in water is expressed as pH. Lower pH means more hydrogen in the water. In some regions low streamwater pH promotes formation of methylmercury (toxic form of mercury), and release of other heavy metals (Grigal 2003). Herbicides and, less frequently, insecticides are sometimes used in forest management regimes. Although use of these chemicals in forestry is relatively low compared to other land-use activities there are prudent practices available to minimize their introduction to streams and lakes (see section on BMPs).
Biological Water Quality Parameters
Important biological measures of water quality in streams include DO, biochemical oxygen demand (BOD), macroinvertebrate community measures, and various measures of fish and aquatic plant communities.
Dissolved oxygen is a chemical measure of water quality but is closely related to biological activity and oxygen demand. Photosynthesis by aquatic plants during daylight releases oxygen into water. Respiration consumes oxygen. This can result in daily and seasonal fluctuations in DO so that concentrations tend to be at their lowest early in the morning. Leaves, needles, or logging slash can be decomposed through chemical and biological oxidation. The oxygen load this creates is referred to as the BOD.
Macroinvertebrates are increasingly used to evaluate water quality. These organisms include aquatic insects like stoneflies, caddisflies, and mayflies, and in some cases, crawfish. Stream condition and water quality are sometimes assessed using measures such as total number, population diversity, and presence or absence of sensitive indicator species. Fish communities are measured with similar metrics and condition factors (health of individual fish). Aquatic plants are assessed either with population measures (diversity indices or favorable/unfavorable species) or with indirect measures such as chlorophyll concentration. For additional reading on biological- and all water-quality parameters see MacDonald et al. (1991).
Controlling Impacts of Forest Management on Water Quality
Harvesting trees and other forest management activities such as road building and site preparation have potential to negatively affect water quality. For this reason, management practices have been developed that can minimize water quality impacts. These practices are known as Best Management Practices (BMPs) and include treatments such as mulching and seeding of road cuts and fills (exposed soil), forested buffers and streamside management zones to maintain shade and wood recruitment near streams, water bars (earthen mounds that divert water) on skid trails, and adequately sized and spaced culverts for roads to move water off the roadway and avoid severe erosion.
Olszewski and Jackson (2006) summarized six principles that can be used to design effective BMPs for forestry: 1) minimize bare ground and soil compaction; 2) separate disturbed soil from surface water; 3) separate fertilizer and pesticide applications from surface water; 4) inhibit hydraulic connections between bare ground and surface water; 5) provide a forested buffer around streams; and 6) engineer stable road surfaces and stream crossings.
Hewlett (1979) conducted a paired-watershed study in the Piedmont Region of the South, looking at impacts of contemporary forest harvesting on water quality. Although impacts were modest compared to other land uses, Hewlett concluded that sediment losses from the watershed could have been reduced by 90 percent if just three changes in the treatment had occurred: 1) adequate buffers, 2) better constructed and maintained roads, and 3) hand planting of seedlings to avoid disturbance of gullies left from past agricultural practices. Williams et al. (2000) conducted another paired-watershed study in the Piedmont Region, but this time with the BMPs that Hewlett recommended. They found, as predicted, that first-year sediment losses were about 90 percent less than those observed by Hewlett in his earlier study. Similar findings have been made about the effectiveness of BMPs to maintain desirable stream temperatures and DO levels, reduce sediment and nutrient loads in streams, and minimize the concentration of herbicides and insecticides in streams.
The Alsea Watershed Study in coastal Oregon (Moring 1975) tested the effectiveness of riparian buffers. One watershed was completely clearcut with logs and slash felled into the stream. A second watershed had patch cuts but riparian buffers were maintained around fish-bearing reaches. A third watershed was monitored as a control. Water temperature significantly increased and DO concentration in the streamwater significantly decreased after clearcutting the unbuffered stream. Only minor changes in temperature and DO concentration occurred for the patch cut and buffered stream. This study demonstrated that management choices can greatly modify water quality impacts.
There are often several BMP options available to achieve a water quality goal. An example is control of drift into a stream from aerial application of herbicides. One way to control drift is to create wide spray buffers (separate pesticide from surface water). However, by using application conditions that create larger drops (fewer fine drops), releasing material at a lower height, or applying when wind direction and wind speed are more favorable, similar reductions in drift to a stream can be achieved.
Under the U.S. Federal Clean Water Act, states are responsible for developing programs to control diffuse, widespread sources of pollution (called nonpoint sources) such as forest management activities. Many state forestry nonpoint-source assessments have concluded that the most important factor determining success of these pollution control programs is implementation of BMPs. Many states have ongoing surveys to determine the level of BMP compliance for forestry activities. The National Association of State Foresters regularly reports on these nonpoint-source-control programs. For additional reading on BMPs see Ice (2004) and Ice and Stednick (2004).
How Natural Disturbances and Environmental Variations affect Water Quality
Although forests create very favorable watershed conditions through development of porous soils and a protective organic forest floor, natural disturbances and variations in environmental conditions can result in water quality that varies dramatically. Some important disturbances include wildfire, insect outbreaks, windthrow, and floods. Forest watersheds and their aquatic communities have adapted to natural disturbances. For example, Kirchner et al. (2001) found that long-term erosion rates from forested watersheds in Idaho were an average of 17 times higher than measured by recent stream gauging station records. Extreme runoff events, coupled with wildfires, can result in infrequent but large impacts to water quality. Insect outbreaks in the forest can dramatically increase N concentrations in adjacent streams. This may be a result of increased mineralization of organic matter and reduced nutrient uptake, or direct deposition of insect frass (feces) into the water (see Ice and Schoenholtz 2003).
Although water quality from forests is generally higher than other land uses (Brown and Binkley 1994), there are exceptions. For example, sediment yields from forested watersheds in northern California are exceptionally high. This results, in part, from very steep, tectonically (earthquake) active conditions with erodible sedimentary rocks and high rainfall rates (Anderson et al. 1976). In northwestern Oregon high P concentrations in streams have been tied to watersheds underlain by prehistoric flood deposits high in P (Kelly et al. 1999).
In some cases what are viewed as undesirable water quality conditions may ultimately be beneficial. Landslides can deliver essential wood and gravels to streams and help to form off-channel habitat. Nutrients that stimulate aquatic plant growth may fuel aquatic macroinverebrate- and ultimately fish productivity. Forest management practices need to avoid unacceptable impacts through use of BMPs while accepting some diversity in water quality conditions that reflect natural variability.
References and Further Reading
Anderson, H.W., M.D. Hoover, and K.G. Reinhart. Forests and Water: Effects of Forest Management on Floods, Sedimentation, and Water Supply. General Technical Report PSW-18. Berkeley, CA: USDA Forest Service Pacific Southwest Research Station. 1976.
Beschta, R.L. “Conceptual models of sediment transport in streams.” In Sediment Transport in Gravel-bed Rivers. Thorne, C.R., J.C. Bathurst, and R.D. Hey, Editors. John Wiley and Sons, Ltd. 1987. pp. 397-419.
Brown, G.W. Forestry and Water Quality. Corvallis, OR: Oregon State University Book Store, Inc. 1980.
Brown, T.C., and D. Binkley. Effect of Management on Water Quality in North American Forests. General Technical Report RM-248. Fort Collins, CO: Rocky Mountain Forest and Range Experiment Station, USDA Forest Service.
Goldman, C.R., A.D. Jassby, and S.H. Hackley. “Decadal, Interannual, and Seasonal Variability in Enrichment Bioassays at Lake Tahoe, California-Nevada, USA.” Canadian Journal of Fisheries and Aquatic Sciences 50(1993):1489-1495.
Grigal, D.F. “Mercury Sequestration in Forests and Peatlands.” Journal of Environmental Quality 32(2003):393-405.
Hewlett, J.D. Forest Water Quality: An Experiment in Harvesting and Regenerating Piedmont Forests. Athens, GA: University of Georgia School of Forestry Press. 1979.
Ice, G.G. “History of Innovative Best Management Practice Development and its Role in Addressing Water Quality Limited Waterbodies.” Journal of Environmental Engineering 130(6):684-689. 2004.
Ice, G.G., and S.H. Schoenholtz. “Understanding How Extremes Influence Water Quality: Experience from Forest Watersheds.” Hydrological Science and Technology 19(1-4):99-107. 2003.
Ice, G.G., and J.D. Stednick, Editors. A Century of Forest and Wildland Watershed Lessons. Bethesda, MD: Society of American Foresters. 2004.
Kelley, V.J., D.D. Lynch, and S.A. Rounds. Sources and Transport of Phosphorus and Nitrogen during Low-Flow Conditions in the Tualatin River, Oregon, 1991-1993. Water-Supply Paper 2465-C. Reston, VA: US Geological Survey. 1999.
Kirchner, J.W., R.C. Finkel, C.S. Riebe, D.E. Granger, J.L. Clayton, J.G. King, and W.F. Megahan. “Mountain Erosion over 10 Yr., 10 K.Y., and 10 M.Y. Time Scales.” Geology 29(7):591-594.
MacDonald, L.H., A.W. Smart, and R.C. Wissmar. Monitoring Guidelines to Evaluate Effects of Forestry Activities on Streams in the Pacific Northwest and Alaska. EPA 910-91-001. Seattle, WA: Region 10, U.S. Environmental Protection Agency. 1991.
Moring, J.R. The Alsea Watershed Study: Effects of Logging on the Aquatic Resources of Three Headwater Streams of the Alsea River, Oregon: Part II—Changes in Environmental Conditions. Research Report No. 9. Corvallis, OR: Oregon Department of Fish and Wildlife. 1975.
Olszewski, R., and R. Jackson. Best Management Practices and Water Quality. Technical Bulletin. Research Triangle Park, NC: National Council for Air and Stream Improvement, Inc. 2006.
Williams, T.M., D.D. Hook, D.J. Lipscomb, X. Zeng, and J.W. Albiston. “Effectiveness of Best Management Practices to Protect Water Quality in the South Carolina Piedmont.” In Tenth Biennial Southern Silvicultural Research Conference Proceedings. General Technical Report SRS-30. Ashville, NC: Southern Research Station, USDA Forest Service. 2000.
Posted: April 2007
Updated: 22 April 2007
|
