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

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Table 3. 1, continued. Published road surface erosion rates in the original units and converted to kilograms per square meter per year. Reference Location Traffic, slope Erosion Rate Reported Sed. Product. (kg m-2 yr) Blong and Humphreys (1982) Papua, New Guinea 70.0 mm/yr 100 Megahan et al. (1983) Idaho 11 mm/yr 16 Riley (1988) New South Wales, Australia 2.4 -3.9 mm/yr 3.6 -5.8 Fahey and Coker (1989, 1992) Smith and Fenton (1993) New Zealand Unvegetated, granite 5.2 -15 Megahan et al. (2001) Idaho Cover density 0.1 -89% 55 -104% gradient 0.1 -247.6 t/ha yr 0.01 -25 Filsslopes Bethlahmy & Kidd (1966) Idaho Unvegetated fillslope 94 t/ha 10.5 mo 11 Megahan (1978) Idaho 12 yr old fillslope 12 t/ha yr 1.2 Fahey and Coker (1989, 1992) Smith and Fenton (1993) New Zealand 55% gradient, variable cover density 0.1 -1.2 the stream channels. Many older roads are located imme- diately adjacent to stream channels or other water bod- ies, and this increases the likelihood that road runoff and erosion will reach the stream channels. Roads impinging on streams can also increase bed and bank erosion by re- ducing channel sinuosity (Schumm, 1971). Newer road designs include vegetative filter strips, more frequent drainage, outsloping of the road surface to disperse road runoff, and narrower road surfaces to reduce the size of the road tread, cutslopes and fillslopes. Whenever pos- sible, roads are being placed in upslope or ridgetop posi- tions rather than in the valley bottom, and this should substantially reduce the potential for adverse effects. 3.3. Stream Temperature Disturbance or removal of the streamside vegetation will generally increase the amount of direct solar radiation reaching the surface waters and potentially increase wa- ter temperatures. Higher water temperatures will directly affect aquatic life, while an important indirect effect of higher water temperatures is the decreased solubility of dissolved oxygen. Surprisingly few recent studies have been published on the effects of forest management practices on water temperature (Beschta et al., 1987; Binkley and Brown, 1993a; Swank and Johnson, 1994). To the best of our knowledge, there are no published data from Colorado assessing stream temperature changes in response to forest management. The dominant mechanism for stream temperature in- creases is the increased exposure of small streams to di- rect solar radiation. Other potential mechanisms include increased air temperatures, channel widening, soil water temperature increases, and streamflow modifications (Ice, in press). Small streams with lower flow rates and shallower flow depths are more susceptible to heating, but they also recover more rapidly (Andrus and Froe- hlich, 1991; Ice, in press). Maintaining streamside shade by retaining some or all of the streamside vegetation can minimize stream temperature increases. The experimen- tal harvests at Coon Creek and Fraser Experimental For- est generally did not expose the channel to direct solar radiation, so the temperature changes induced by forest harvest are expected to be negligible (C. Troendle, Mat - com Corp., pers. comm., 2003). P Most aquatic organisms have optimal temperature ranges and temperature increases of more than 2 °C may alter development rates of aquatic organisms. An in- crease in sunlight and a corresponding increase in water temperatures will increase primary productivity and the growth rates of aquatic organisms. In colder high- eleva- tion streams, an increase in stream temperature might be considered beneficial in terms of increasing productivity, particularly if the stream is used for recreational fish- ing. A comparable increase in stream temperatures in a lower - elevation stream might be considered detrimental, and this indicates the importance of defining the loca-