In situ size distribution of suspended particles in the Fraser River

Size distributions of suspended sediment particles under low flow condition s in the Fraser River were measured using a submersible laser instrument. B y comparing the in situ size distribution measured using this instrument wi th the size distribution of primary particles (measured by collecting sedim ent samples and analyzing them for size distribution after dispersing the p articles by ultrasonic vibration), it was concluded that suspended sediment particles in the Fraser River downstream of pulp mill effluent outfalls ar e transported as agglomerations of particles (flocs) rather than as individ ual particles. Flocculation of river sediments in estuaries has been invest igated in earlier studies, which show that saltwater intrusion contributes to the flocculation mechanism. Freshwater flocculation, such as the one obs erved in the present study, also has been reported in the literature, and i t is attributed to the presence of organic materials and other contaminants from industrial and sewage treatment plant effluents. This paper describes the details of the submersible laser instrument and its use in the Fraser River, British Columbia, Canada.