Sediment accumulation rates in the lower Columbia River

Sediment accumulation rates in the lower Columbia river have been determined by three radiometric techniques: (1) 154Eu/152Eu activity ratios as a function of depth in the sediment column; (2) location of the 239240Pu activity maximum resulting from fallout input during atmospheric nuclear device testing in the period 1961-63; and (3) the inflection in 238Pu/239240Pu ratios signifying the arrival of debris from the aborted Transit Navigational Satellite (SNAP-9A). Using these techniques, calculated accumulation rates behind McNary Reservoir, the first slack water site downstream of the Hanford Reservation, appear far lower than previously thought. Long-term sediment storage occurs in all reservoirs sampled in the lower Columbia. These findings do not support previous claims that sediment accumulation at these sites is transitory. Moreover, virtually all of the suspended material carried by the mainstream Columbia below its confluence with the Snake river may be exported to the northeast Pacific Ocean. (author)