Forests and Water: A State of the Art Review for Colorado

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Chapter 3 Effects of Forest Management Activities and Fire on Water Quality 3.1. Introduction Water from forested watersheds is typically higher in quality than waters draining from any other major land use (USEPA, 1980). The natural nutrient cycling and streamflow generation processes in forested areas typi- cally result in water that is low in nutrient and suspended sediment concentrations. Forested watersheds are often the source of public drinking water supplies, and this is particularly true in Colorado due to the higher water yields from forested areas (Chapter 2) as well as the high quality of the runoff. The potential effects of forest management activities on water quality are often of greater concern than the changes in runoff. Numerous studies have shown that forest management activities may alter the physical and chemical characteristics of runoff. Parameters of particular concern include turbidity, total suspended solids, temperature, and nutrients. Forest management also can affect the amount and transport of larger -sized particles (bedload). Changes in these parameters can af- fect the beneficial uses of water and aquatic ecosystems (MacDonald et al., 1991). A change in the chemical and physical characteristics of water, especially when the water is being used for domestic purposes, may necessitate changes in water treatment and purifica- tion. Increases in other parameters, such as temperature or bedload, may be of less concern for drinking water providers, but may be of greater concern for fisheries or other aquatic resources. Large increases in sediment loads will accelerate the rate of reservoir sedimentation and can initiate a series of changes in channel morphol- ogy through channel widening, or increase the likelihood of flooding, which in turn can adversely affect people and aquatic resources. There is an extensive literature on the effects of forest management on water quality, but only limited informa- tion from Colorado. A review of the literature indicates considerable variability with respect to the effects of for- est management on water quality. This can be attributed to differences in the type and location of management activities as well as site - specific differences in runoff and erosion processes. This chapter will review the po- tential effects of forest management activities and fire on erosion and sedimentation, stream temperature, and 21 nutrients. It should be recognized that other activities in forested watersheds, such as urbanization, water diver- sions, gravel mining, and hardrock mining, may have a greater effect on water quality and aquatic resources than forest management. Because these other activities represent a change in land use rather than forest manage- ment, they fall outside the scope of this report and will not be discussed here. The effects on water quality of other types of forest disturbance, such as insects and dis- ease, are not well documented, but the likely effects can be extrapolated from the principles and data presented in this chapter. 3.2. Erosion and Sedimentation Soil erosion is defined as the detachment and move- ment of soil particles. The site characteristics that have the greatest effect on erosion rates include the amount and type of precipitation, vegetative cover, soil texture, and slope (Renfro, 1975; Falletti, 1977; Renard et al., 1997). Undisturbed forested watersheds typically have very low erosion rates because of the high infiltration rates and limited surface runoff (Binkley and Brown, 1993a, 1993b). Erosion rates have been estimated as less than 0.1 tons per acre per year for most forested areas in the interior western U.S. (Patric et al., 1984). Similar or lower values have been documented for different forest types in Colorado (Leaf, 1970; Gary, 1975) and for pon- derosa pine -mixed conifer forests in northern Arizona (Brown et al., 1974). Average long -term erosion rates can be substantially higher where gullying, debris flows, or other types of mass movements are important erosion processes (Gary, 1975; Bovis, 1978). Undisturbed for- ested areas in Colorado typically have very low erosion rates because much of the precipitation falls as snow, infiltration rates are high, and mass movements are ei- ther infrequent or relatively inactive. In the lower- and mid- elevation forests extreme rainstorms can generate much higher sediment yields. Sediment yields of up to 6 tons per acre were recorded from a small, minimally- disturbed ponderosa pine watershed in northern Arizona that was subjected to a 100 -year rainstorm (Brown et al., 1974). Site or soil disturbance by forest management activities will generally increase soil erosion. Greater site distur- bance is usually associated with a greater increase in