Abstract: Following the flooding of February 1996, the North Santiam River below Detroit Reservoir carried an extraordinarily high level of turbidity that lasted for months. The Willamette National Forest, Pacific NW Research Station, Oregon State University, and the City of Salem undertook a joint effort to determine the composition and source of the persistent turbidity.
The primary causes of the persistent turbidity were smectite clays. These constituents were found to have multiple sources in broadly identifiable locations throughout the North Santiam River subbasin. The study reaffirms earlier findings that earthflows and other erosion processes affecting deeply weathered, smectitic deposits of the ancestral Western Cascades are the main sources of these materials in rivers and streams. These deposits are present, but less prevalent, in the younger High Cascades.
Streamflow in subsequent storm events has been of lesser magnitude and has not resulted in significant modification of channel patterns established during the February event. Absence of severe persistent turbidity subsequent to the flood of February 1996 may indicate that the river channels were effectively winnowed of the major smectitic deposits. Strategies for minimizing the production and effects of persistent turbidity include avoidance of earthflow deposits, restoration of disturbed sites, maintenance and enhancement of riparian ecosystems, construction of stream structures to accommodate floods and changes in reservoir operations.