Recent evolution of the Outardes Estuary, Quebec, Canada: consequences of dam construction on the river



The Outardes Estuary (Quebec) is a regressive sandy system developed in a sub-boreal climatic setting. The Outardes River drains an area of 18 780 km2 and was dammed for hydroelectric purposes in the 1960s and‘70s. By reconstructing the hydrographs and mapping the morphology of the estuary from aerial photographs over a period of 21 years, we have documented the changes in the estuary resulting from the modification of the hydrologic regime.

Until the mid-1960s, a catastrophic spring freshet (1800–2800 m33s-1) controlled the morphology of the estuary. The mouth was dominated by a disorganized, braided channel pattern, and large fluvial discharge-generated transverse bars. Peripheral channel areas in the upper estuary were zones of transport at flood stages. For 18 months in 1968 and‘69 the river discharge was drastically reduced (50 m3s-1) to accomplish the main reservoir filling. Tidal currents and waves filled and remodelled the morphology of the mouth of the estuary, while the peripheral zones in the upper estuary experienced low-energy sedimentation. Since the generating plants were brought on-line in 1969, the fluvial discharge has been unable to modify substantially the sand body geometry in the mouth of the estuary. In the upper estuary the peripheral regions continue to be filled in, but the thalweg is being eroded.

Based on the results of our coring programme, we suggest that major environmental changes may leave a sedimentary signature (addition or subtraction of size modes) which can be identified by the detailed analysis of grain size distributions.