River channels and estuaries sometimes become obstructed by accumulations of silt or by submerged objects. This can result in the blockage of transportation routes or even in flooding. A dredger is a self-powered barge-like vessel that is used primarily to clear these waterways. It is also used to construct dams and canals for drainage projects, for the recovery of submerged objects, or to extract valuable minerals, such as gold, from silt deposits.

The first known dredgers were in use in the Netherlands during the twelfth century. For many centuries, the Dutch have used bottom material to build dikes. The seawater behind the dikes is drained using windmills, forming polders. Then the new land can be desalinized and used for agriculture. The first steam-powered dredger was invented in England in 1796. Scottish engineer, John Rennie, made improvements to it, and used them in the construction of the London Docks in 1804 and 1805.

Dredgers can be secured to the bottom with the use of pinions, called spuds. The material at the bottom is removed with either a scoop shovel, grab bucket, or bucket ladder, and is deposited either on an adjacent barge or directly on the shore. Some hydraulic dredgers even use hoses to suck matter from the bottom, similar to a vacuum cleaner. Modern dredgers are powered by diesel fuel, though steam is still used in some areas. They are commonly seen in river systems, harbors and estuaries in the United States and other parts of the world.