The U.S. marine transportation system encompasses a national and global network of navigable ocean, lake, river, and inland waterway routes; the vessels that carry waterborne commerce; a complex of ports and terminals serving as inter modal points of transfer between the water system and the land-based transportation modes; ship operators; an extensive supplier base; and shipboard, shipyard, and long shore labor forces. The system includes over 3,500 bulk oil transfer facilities, 10,000 marinas, 18,000 bridges, a network of locks and dams, and 97,000 aids to navigation. Diverse groups of waterway users commercial vessels making 70,000 port calls annually, 110,000 fishing vessels, and 20 million recreational vessels vie for access to our waterways often with conflicting interests and purposes. Additionally, the general public is concerned about the waterways environmental health and esthetics.

Source: http://www.yoto98.noaa.gov/