When air, water and soil pollution end up at the bottom of a river, they usually stay relatively put unless stirred up. This is done on small scale whenever a large boat anchor puts down in a harbor without mooring. This may also be done when constructing docks or platforms for extraction or recreation. However, the most common way these waters are disturbed is through dredging operations that keep channels open that would otherwise be narrowed and made to shallow for ship traffic by the buildup of silty erosion.
For this reason, many waterways were declared Super fund sites in the US during the 1990s, with special monies made available for the removal and treatment of these sediments, at great cost. Many of the major waterways in North America have been found to contain large amounts of heavy metals, PCBs and other persistent pollutants.
