Roughly one-sixth of the intercity cargo in the U.S. travels on the nation’s 25,000 miles of commercially active inland and intracoastal waterways. Almost half, or 12,000 miles, are federal waterways known as the Inland Waterway System, which is managed by the Army Corps of Engineers. Barge operators pay a diesel fuel tax to help fund work on the federal system covering 38 states and includes about 240 locks.

The inland waterway system is especially important in moving bulk commodities such as coal and grain.

U.S. seaports handle nearly 80 percent of the volume of U.S. merchandise trade with other countries, according to the International Trade Administration. The American Association of Port Authorities counts some 360 commercial ports comprising some 3,200 cargo and passenger handling facilities.

The Maritime Administration said customs ports handled 29.5 million 20-foot equivalent units — a shipping container — and 244 million metric tons of goods in 2012, according to the latest data available. The Los Angeles and Long Beach ports in California, New York and Savannah, Ga., were the four biggest ports for both types of shipping.