44 2033180199

Particle trapping effects by dams on downstream deposits in the Saguenay River Prodelta: textural data from an unbioturbated core sample.

Charles T. Schafer

Unbioturbated sediment deposits present on the Saguenay River Prodelta (North Arm, Saguenay Fiord) indicate a statistically-significant 8.4µm decrease in average Median Diameter (Md) in a post-dams period (~1965 – 1970) compared to a pre-dams period (~1907 - 1912).  The post-dams average percentage of the 2.3ǿ, 2.7ø, 3.0ø, 3.7ø, and 4.0ǿ fine and very fine sand-size classes decreases, whereas that of the 4.7ø, 5.0ø, 5.3ø, 5.7ø, 6.0ø and 6.3ǿ coarse, medium and fine silt-sizes increases in relation to those sediments deposited in pre-dams time.  Although cumulative effects of climate change and dam construction on river hydrology and prodelta sediment texture variations remain incomplete for the Saguenay River system as a whole, it appears that, for the river's lower reaches, the post-dams period average reduction in Maximum Mean Monthly Discharge (MMMD) that is typically associated with the river’s spring freshet, has shifted the deposition of relatively fine silt-size particles toward the upstream end of the river's Prodelta.  This shift is associated with a 0.52 cm yr-1 increase in average sedimentation rate based on 210Pb dating results.

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