However, the harm has to be balanced against the damage that can be done by a slow, gradual release of toxics that extends over many years, if the sediments are left in place. The bucket or clamshell dredge is the most widely used dredge in the Great Lakes. Its two hinged halves are opened wide and then dropped onto the bottom where they sink into the sediment. The operator then raises the dredge, enclosing a load of sediment. The closed bucket is then raised, swung over a barge, and opened, allowing the sediment to drop onto the barge.
Bucket dredges are excellent for use in close quarters, such as around docks or breakwaters.
Their main disadvantage is that sediment can spill out of the top of the bucket as it is raised up. The watertight bucket uses covers on top of the bucket to minimize spillage. In essence, hydraulic dredges are enormous vacuum cleaners that suck sediments from the bottom. They may be equipped with rotating blades, augers, or high-pressure water jets to loosen the sediment. The most common type in use in the U.S. is called a cutterhead, which uses rotating blades to loosen sediments.