Two modern canals that have been especially important to waterborne commerce of the United States are the Panama Canal and the St. Lawrence Seaway. The Panama Canal, opened in 1914, connects the Pacific and Atlantic Oceans via the narrow isthmus of Panama, saving thousands of miles of travel around South America. The St. Lawrence Seaway, opened in 1959, connects the inland Great Lakes of the United States and Canada with the Atlantic Ocean via the St. Lawrence River. Both of these waterways are considered vital arteries of commerce for US foreign trade. But today, evolving shipping technologies on both land and sea and changes in world trade patterns raise questions about the long-term role of both waterways. The limiting lock dimensions of the two canals are becoming an increasingly important factor in the role each waterway plays in world trade. The rapid increase in the trend toward larger vessels, the opening of a trans-isthmus pipeline, and the use of double-stack container trains has siphoned off high-value traffic. It appears that as the average vessel size in the world fleet continues to grow, the percentage of the fleet able to transit each waterway will continue to decline. Enlarging either system to handle larger ships would be a very expensive undertaking and would also raise a host of environmental issues, so ultimately both waterways seem likely to play a diminished role in world trade.