Dredging is the underwater excavation of a placer deposit by floating equipment.

Dredging systems are classified as mechanical or hydraulic, depending on the method of material transport.

The bucket-ladder, or bucket-line, dredge has been the traditional placer-mining tool, and it is still the most flexible method for dredging under varying conditions. It consists of a single hull supporting an excavating and lifting mechanism, beneficiation circuits, and waste-disposal systems.

The excavation equipment consists of an endless chain of open buckets that travel around a truss or ladder. The lower end of the ladder rests on the mine face—that is, the bottom of the pond where excavation takes place—and the top end is located near the centre of the dredge, at the feed hopper of the treatment plant.

The chain of buckets passes around the upper end of the ladder at a drive sprocket (called the upper tumbler) and loops downward to an idler sprocket (the lower tumbler) at the bottom. The filled buckets, supported by rollers, are pulled up the ladder and dump their load into the hopper.

After the valuable material has been removed by the treatment plant, waste is dumped off the back end of the dredge.

The clamshell dredge, another mechanical system, is characterized by a large single bucket operating at the end of cables.Although it can operate in deeper water than other systems and handles large particles and trash well, it has the disadvantage of being a discontinuous, batch-type system, taking approximately one bite per minute.

In pure hydraulic dredging systems, the digging and lifting force is either pure suction, suction with hydrojet assistance, or entirely hydrojet.

They are best suited to digging relatively small-sized loose material such as sand and gravel, marine shell deposits, mill tailings, and unconsolidated overburden.

Hydraulic dredging has also been applied to the mining of deposits containing diamonds, tin, tungsten, niobium-tantalum, titanium, monazite, and rare earths.

The digging power of hydraulic systems has been greatly increased by the addition of underwater cutting heads.

The cutter suction dredge has a rotary cutting head or other excavating tool for loosening and mixing soil at the face of the mine.

The material falls downward to the mouth of a centrifugal pump, and this transports the slurry (containing 20 to 25 percent solids) to the processing plant. Normally, the dredge is held in place during cutting by a pile called a spud. Winches and wire ropes are used to swing the dredge in an arc around the spud until all the material in the arc has been removed.

The dredge is then moved ahead and the process repeated. The cutter suction dredge is most suitable for mining softer deposits where the material is of a relatively low specific gravity or fine particle size—for example, in sand and gravel pits, phosphate mines, and various salt deposits.