Sedimentation, mercury contamination, and clay mineralogy of the Dorena Lake watershed, western Oregon

Dorena Lake, constructed in 1949, is a flood-control reservoir with a 686 km2 watershed in the western Cascade Mountains of Oregon. From the stratigraphy of eight sediment cores and measurements of sediment thickness, the total volume of lake sediment is 3 million m3, and the average sediment yield for the first fifty years of reservoir operation is 85 tons/km2/year. Yields during that period vary from 35 to 434 tons/km2/year and appear to be controlled primarily by storm magnitude and frequency. The variation in yield that might be produced by changes in logging rates or methods is too small to be detected in this basin because the effects of localized events tend to be averaged out and sediment delivery ratios are generally low in large watersheds.; Cinnabar mining in the nearby Cottage Grove Lake watershed and mercury amalgamation used in gold mining in the Dorena Lake watershed have created mercury contamination problems in the fish of both reservoirs. On average, Cottage Grove Lake sediment has ten times the mercury concentration of sediment in Dorena Lake (2.720 ppm versus 0.242 ppm). Mercury content in Cottage Grove Lake sediment shows a sharp initial decrease and leveling off with time that reflects the end of the major cinnabar mining phase. In Dorena. Lake, mercury input remained fairly constant with time, but small increases in mercury are associated with the deposits of large floods.; The clay mineralogy of bedrock and soil varies systematically across the Dorena Lake watershed, and the three major stream tributaries carry sediment with distinct mineralogical signatures. Where smectite is abundant, topographic slopes are more gentle, and both debris flows and earthflows are common. In the Bohemia Mining District where smectite is lacking, debris slides and flows are the dominant mass movement processes, earthflows are absent, and slopes are markedly steeper. Clay mineralogy appears to be integrally tied to the hillslope processes operating within the watershed and thus affects the geomorphology of the landscape.; This dissertation includes both my previously published and co-authored materials.