The Salmon Creek Watershed drains 325 km2 of forested terrain in the Cascade Mountains of western Oregon. Over a 30–year period (from 1955 to 1984) average daily maximum and minimum stream temperatures, calculated from the 10 warmest days of each year, have risen 6°C and 2°C, respectively. In contrast, a small decrease in maximum air temperatures was found over the same period. Regression analysis indicated a highly significant (p < 0.01) relationship between a cumulative index of forest harvesting and maximum stream temperatures. Maximum temperatures also tended to increase for several years following major peak flow events. The interaction between harvest activity (logging and road construction), changing forest and riparian management practices and the occurrence of natural hydrologic events (peak flows and associated mass soil movements) tend to obscure specific cause‐and‐effect relationships regarding long‐term changes in maximum stream temperature.
Streamside red alder (Alnus rubra Bong.) stands are common in western Oregon, and they have been suspected of causing water quality problems in domestic supplies during autumn leaf fall. Studies in the Seaside municipal watershed showed potential water quality effects (particularly increased color) from alder leaves, but stream sampling during 1981–82 revealed no chronic problems. The few observed short‐term increases in water color occurred near the onset of storm flows, which suggested a flushing of organic matter storage sites. An extended period of unusually low flows and high leaf fall are probably necessary to produce significant water quality problems in this stream system. Laboratory leaching of alder leaves in filtered stream water indicated a fairly constant release of colored organic matter over time, and running water leached this matter more efficiently than still water. Water color increased linearly with increasing leaf mass added to still water, and for a given leaf mass there appeared to be a limit to the amount of colored matter that could be removed in the first 48 hours of leaching. Other laboratory tests showed that ultraviolet absorbance (254 mm) may provide a reasonable estimate of dissolved organic carbon concentrations in systems dominated by alder leaf inputs.
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