Portland Harbor, located between the cities of Portland and South Portland, is situated in Casco Bay on the southwestern coast of Maine, about 50 miles northeast of the Maine-New Hampshire border. It is strategically located, being the nearest American deepwater port to Europe, and serves as a key center for shipping by both land and sea. Imports primarily include crude petroleum and chemicals, while exports include wood pulp and petroleum products. Portland Harbor is an important economic and commercial link to a vast territory extending northward to Canada. Rail lines and petroleum pipelines connect Portland’s terminals with Quebec and Montreal.

The main harbor is located on the Fore River, which separates Portland and South Portland. A secondary harbor, used primarily by pleasure craft, is located in Back Cove, a relatively shallow cove on the north side of the peninsula, where the industrial and commercial heart of Portland is located.

Initial work in Portland Harbor took place between 1836 - 1874. It included construction of an eight-foot channel into Back Cove, a 20-foot channel through the Fore River entrance bar, and a breakwater on the south side of the Fore River entrance, generally parallel to the Portland waterfront. The area immediately south of the breakwater was later filled to form a major part of the South Portland waterfront.

The most recent work in Portland Harbor was completed in 1968. It consists of:

• A 9,000-foot-long entrance channel, 45 feet deep and 1,000 feet wide, extending from the southwestern end of Casco Bay (between Cushing Island and South Portland) to a point between House Island and Spring Point.
• A 45-foot-deep turning basin and anchorage, 170 acres in area, starting at the end of the 45-foot-deep entrance channel and extending for one-half mile around Spring Point.
• A three-mile-long, 35-foot-deep channel extending from the end of the 45-foot-deep turning basin and anchorage, up the Fore River and past the Portland Bridge (Route 77), and ending at a point 700 feet before Veterans Memorial Bridge (U.S. Route 1). The channel is more than 500 feet wide at the turning basin, and gradually narrows at it approaches the Portland Bridge. From the Portland Bridge to the site of the former Vaughan Bridge (about 6,800 feet), the channel is 400 feet wide. From that point to its upstream end (500 feet), the channel is 300 feet wide.
• A 35-foot-deep turning basin on the southern side of the 35-foot-deep channel, at a point opposite Amoco Wharf, 1,000 feet downstream of Veterans Memorial Bridge.
• A 30-foot-deep anchorage, approximately one mile long, east of Fish Point. The anchorage begins at the junction of the 45-foot entrance channel and 45-foot turning basin, and bends around Fish Point toward Back Cove.
• A channel, approximately 1.2 miles long, extending from the Fish Point Anchorage to a point inside Back Cove, about 2,500 feet south of Tukey Bridge (U.S. Route 295). From the Fish Point Anchorage to the Canadian National Railroad (Grand Trunk) Bridge (a distance of about 2,500 feet), the channel is 30 feet deep and 300 feet wide. From the railroad bridge to the Tukey Bridge (1,700 feet), the channel is 14 feet deep with varying widths. From the Tukey Bridge and running along 2,500 feet of the east side of Back Cove, the channel is 12 feet deep and 300 feet wide.
• A 2,000-foot-long stone breakwater on the southerly side of the mouth of the inner harbor, near Spring Point.
• A 900-foot-long stone breakwater from Spring Point to Spring Point Ledge Lighthouse.
• The maintenance of Soldier Ledge Channel at Hussey Sound to a depth of 40 feet. The Corps originally removed two ledges that were obstructing the main channel. Hussey Sound is a passage between Peaks and Long islands, about three miles northeast of Fore River.