Riparian shade and stream temperature; an alternative perspective.

For streams in the upper Columbia River Basin, and elsewhere across much of the Intermountain West, the important role of shade with respect to stream temperatures has become an increasingly important issue. For example, the recent discussion of riparian systems by Larson and Larson (Rangelands, August 1996) focused on shade and temperature. However, I found that their perspective left much unsaid concerning the physical processes influencing stream temperatures and the role of riparian vegetation. There is no doubt that the reply that follows will also leave much unsaid, but I trust thalt readers will come away with a greater understanding about the relationship between riparian vegetation, shade, and sitream temperatures. In the Pacific Northwest, decreasing salmon populations over the past decades have resulted in an urgent need for improving the management of watersheds, fish habitat, and water quality. Wild salmon, which once numbered more than 10 million returning adults in the Columbia River basin alone, have declined to less than one-tenth that number up and down the coast of the Pacific Northwest (National Research Council 1996). A vanety of natural factors (ocean conditions, climatic variations) and human factors (fishing, dams, hatcheries, urbanization, forest practices, agricultural practices, livestock grazing, and others) have affected salmon populations. Once salmonids leave the ocean and enter freshwater streams and rivers they are critically dependent upon the quality of the water and instream habii ta ts, particularly for spawning and rearing. Unfortunately, many streams in the upper Columbia River Basin and elsewhere in the Intermountain West have experienced serious long-term declines in the quality of aquatic habitats for both anadromous and resident fisheries (e.g., Mcintosh et al. 1994, Wissmar et al. 1994). Various characteristics of a stream system, the adjacent riparian area, and upslope watershed conditions collectively influence instream habitats. However, a stream's summertime temperature regime is often a critical characteristic of habitat quality. Temperature not only regulates the biological activity of aquatic organisrns (higher temperatures result in increased metabolic rates) but the capability of water to hold oxygen decreases with higher temperatures. With high summertime temperatures, salmonids and other cold water species may experience increased stress levels, greater susceptibility to disease, and an inability to compete with warm water species. Larson and Larson (1996) indicate that the State of Oregon has established forest practices rules and regulations for riparian zones. Indeed, forest practices rules, recently modified to provide increased levels of protection to fisheries and other aquatic organisms (Oregon Department of Forestry 1994), have been in place for approximately twenty-five years. In contrast, comparable levels of water quality and stream protection for agricultural and rangeland streams do not exist in Oregon. A recent compilation by the Oregon Department of Environmental Quality (1996) indicates large numbers of streams and stream miles associated with agricultural and rangeland areas do not meet current water quality standards for temperature.