Sediment and toxicity
Sediment will typically accumulate behind the dam. The quantity depends on factors such as the surrounding geology (e.g. soil vs. rock), how long the dam has been in place, upstream river velocity and the volume of the reservoir that the dam controls. The quality of the sediment will also vary depending on its source, physical characteristics and potential effect from introduced contaminants.
Removing the dam has the potential to release this accumulated sediment that could be transported downstream, impacting various aquatic components by smothering or because of increased turbidity. If contaminated, this will lead to additional toxicity issues. If the river flow and sediment load favour transport, the sediment may be transported across confluences and may impact other rivers, lakes or bays further downstream.
Potentially toxic contaminants may have been buried in the lower layers of the accumulated sediment over time. These buried contaminants may be resuspended when disturbed as part of the decommissioning work. The toxins, previously inert because of burial, may again become bioavailable upon resuspension. This may have significant impact on aquatic organisms as well as terrestrial animals that may be part of the same food chain.
What to do
It is essential that persons proposing dam decommissioning projects ensure that an appropriate level of sediment sampling, analysis and planning is conducted to identify the measures necessary to mitigate the potential for uncontrolled sediment releases once the dam structure has been removed (either partially or completely).
The preparation of a proper sediment management plan that details the proposed sediment mitigation measures, including sediment stabilization methods and techniques, disposal methods (if required) and site restoration methods, is essential in avoiding adverse downstream effects during the decommissioning and for years thereafter.
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