Sedimentation, the natural movement of sand and silt carried downstream, slowly accumulates in channels and harbors. Dredging is therefore commonly used to preserve or expand the depth of navigation routes, anchorages, and berthing zones so vessels can move safely. Ships need sufficient water depth to remain afloat and avoid grounding, and this required depth has grown over time as vessels have become larger. Because these large ships transport most of the nation’s imported goods, dredging is essential to supporting the country’s economy.