Mechanical Dredging is just as the name implies, a giant shovel that mechanically scoops up bottom material and deposits it into barges, often called scows, to be towed away and either bottom dumped in another location or more normally, off loaded with another crane or excavator onto land for recycling or, if contaminated, trucked to a disposal facility designed to contain such contaminants or treated with polymers to encapsulate hazardous contaminants.
Mechanical Dredging usually has a much faster mobilization (setup) time then Hydraulic Dredging. One disadvantage is the spoils (bottom material) scow must be pushed with a Tug Boat or Push-Boat to another location, typically handled at least once again to get unloaded, and often a third time into trucks for road transport.
Both Mechanical and Hydraulic Dredging are capable of excavating most all of the spoils (bottom material) when correctly sized. Machine sizes range from relatively tiny for small slips to absolutely gargantuan, with enough force to fracture and remove solid rock.