MLW (mean low water): The average height of the low waters over a 19-year period.
Source: Adapted from US Army Corp of Engineers (2012) U.S. Port and Inland Waterways Modernization: Preparing for Post-Panamax Vessels.
Note: Containership capacity refers to full ships. A port can accommodate larger ships if they are partially loaded.
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- The Eastern Seaboard, with the exception of the St. Lawrence / Great Lakes system, offers no significant navigable river system as the Appalachian Mountains are just a few hundred kilometers inland. The upper Great Lakes (Erie, Huron, Michigan and Superior) offer good navigation depths, but navigation is limited by the waterways between the lakes and by ice in winter. Further, access to the Atlantic is limited to the depth and lock size of the St. Lawrence Seaway, which is closed for a few months during the winter. The St. Lawrence enables to go deep inland, but maritime vessels can go up to Montreal (draft of 37 feet and the capacity to accommodate ships up to 4,200 TEUs), which is at the same longitude than New York. There are channel depth limitations for accessing several East Coast ports, with many of them limited to ships around the Panamax class (4,500 TEUs) with the exception of Halifax (55 feet), Baltimore (50 feet) and Hampton Roads (50 feet) to enable these ports to accommodate Post-Panamax containerships. Based upon tidal conditions, it is possible for some Post-Panamax containerships in the range of 6,000 TEUs to call East Coast ports within the depth of 45 feet (Post Panamax I). For instance, ships in the range of 7,000 TEUs can call the port of Savannah. The Eastern Seaboard however has also a complex but underused coastal waterway transport system. The Intracoastal Waterway services most of the Eastern Seaboard and consists of a series of bays, inlets, sounds and artificial canals.
- For the Gulf of Mexico, the Mississippi inland waterway system is extensive but limited to depths of less than 15 feet for the most part. Ports have a mean low water depth of less than 45 feet, limiting the containership capacity to less than 6,000 TEUs. Under such circumstances, ports along the Mississippi are dominantly handling barges loaded with agricultural commodities, which implies a highly seasonal traffic (end of the summer and fall). Additionally, the Mississippi system has a north-south orientation while most of the goods flows are east-west, implying a limited potential to service intermodal freight movements. Like the Eastern Seaboard, there is an Intracoastal Waterway ranging from Texas to Florida.
- The Western Seaboard has four major deep water gateways, Vancouver, Seattle / Tacoma, San Oakland and Los Angeles / Long Beach, but poor waterway access to the interior because of the Sierra Nevada and the Rockies. The only exception is the Columbia River basin which is accessible to deep-sea ships up to Portland (draft of 40 feet) which is about 160 km inland. From Portland, container barge services go an additional 575 km inland. Most West Coast ports have the potential to accommodate ships of 12,000 TEUs and above. In late 2015, a Triple E class ship (Post Panamax III) docked at the port of Los Angeles, the largest containership to ever call a North American port. Prince Rupert is the container port having the deepest berth depth and can technically accommodate the largest containerships currently designed.