Water transportation is generally used for the movement of bulk commodities of relatively low value per tonne, such as coal, ore, grain, gravel and salt. Water transportation may be separated into three general categories: ocean transportation, inland water transportation and coastal transportation. Ocean transportation is important to Canada because about one-third of all that Canada produces is exported by ship to customers overseas, carried by large, oceangoing vessels that serve Canada's major ports. Canadian and American lake carriers and other vessels operating on the Great Lakes and in canals and inland waterways supply goods to inland regions of Canada. For example, much of Canada's iron ore moves via the Great Lakes-St Lawrence Seaway system to steel mills in the US. On the return journey, many of these vessels carry coal from US mines. Transportation in coastal waters is also important. Logs, wood chips, lumber and other bulk commodities are moved by barge in BC's coastal waters (see: Forest Harvesting; Ports and Harbours; Shipping Industry).