COLREGS Demarcation Lines Explained

Every time you head out of a U.S. harbor, there's an invisible line where one set of navigation rules stops and another starts. That line is called a COLREGS demarcation line. It's a small topic, but it explains why the Rules of the Road come in two versions, and it shows up in exam questions about what counts as Inland waters.

This guide explains what the lines are, where to find the official descriptions, and a few real examples, all checked against 33 CFR Part 80 and the Navigation Rules published by the U.S. Coast Guard Navigation Center (NAVCEN).

What a demarcation line is

The United States follows two sets of navigation rules:

• The International Rules, formally the International Regulations for Preventing Collisions at Sea, 1972, usually called the 72 COLREGS.
• The Inland Rules, the U.S. Inland Navigation Rules in 33 CFR Part 83.

The demarcation lines decide which set applies where. In the words of 33 CFR 80.01, the regulations "establish the lines of demarcation delineating those waters upon which mariners shall comply with" the 72 COLREGS and "those water upon which mariners shall comply with the Inland Navigation Rules."

The key sentence is short: "The waters inside of the lines are Inland Rules waters. The waters outside the lines are COLREGS waters."

So the lines are drawn across harbor entrances, bay mouths, and inlets. Seaward of the line, you follow the International Rules. Shoreward, you follow the Inland Rules.

How the rules themselves point to the lines

Both rule sets lean on these lines without spelling them out.

International Rule 1(a) says the rules apply "to all vessels upon the high seas and in all waters connected therewith navigable by seagoing vessels." Rule 1(b) then leaves room for special rules made by an appropriate authority for roadsteads, harbors, rivers, lakes, or inland waterways.

The Inland Rules are those special rules. Inland Rule 1(b)(i) says the Inland Rules "constitute special rules made by an appropriate authority within the meaning of Rule 1(b) of the International Regulations." And Inland Rule 1(a) says they apply to all vessels upon the inland waters of the United States, and to vessels of the United States on the Canadian waters of the Great Lakes to the extent that there is no conflict with Canadian law.

The Inland definition of "Inland Waters" in Inland Rule 3 ties it together: "the navigable waters of the United States shoreward of the navigational demarcation lines dividing the high seas from harbors, rivers, and other inland waters of the United States and the waters of the Great Lakes on the United States side of the International Boundary."

Notice the Great Lakes piece. There are no demarcation lines on the Great Lakes because the U.S. side of the Great Lakes is Inland waters by definition.

Where to find the official lines

The lines are listed in 33 CFR Part 80, COLREGS Demarcation Lines, which you can read free on the eCFR. The NAVCEN Navigation Rules also list 33 CFR 80 under their legal citations.

Part 80 is organized by coastline and region, from Maine down the Atlantic coast, around the Gulf of Mexico, up the Pacific coast, and out to Hawaii, the Pacific islands, and Alaska. Each section names a stretch of coast or a specific harbor and describes the line, usually from one charted light or landmark to another, sometimes with latitude and longitude.

Lines are also printed on nautical charts. On a chart, look for a dashed line labeled "COLREGS Demarcation Line" near harbor entrances. If you're unsure, the text in Part 80 is the authority.

One technical note from 80.01(c): coordinates in Part 80 aren't meant to be plotted on NAD 83 charts unless they're expressly labeled NAD 83, or until you apply the correction published on that chart.

Real examples from 33 CFR Part 80

A few examples show how different the lines can be.

Some sections go the other way and say the International Rules apply everywhere in an area:

• Alaska (80.1705): "The 72 COLREGS shall apply on all the sounds, bays, harbors, and inlets of Alaska."
• Puget Sound (80.1395): The 72 COLREGS apply on all waters of Puget Sound and adjacent waters, including Lake Union, Lake Washington, Hood Canal, and all tributaries.
• Hawaii (80.1410): Except for specific harbors listed elsewhere in Part 80, the 72 COLREGS apply on all other bays, harbors, and lagoons of the Hawaiian Islands.
• Santa Catalina Island (80.1102): The 72 COLREGS apply to the harbors on Santa Catalina Island.

That surprises a lot of people. You can be tied up in a Seattle marina and be in COLREGS waters.

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Why it matters on the water

The two rule sets are very similar, but the differences are real, and the demarcation line tells you which version you're under. A few examples:

• Maneuvering signals (Rule 34). Under International rules, one short blast means "I am altering my course to starboard." Under Inland rules, for power-driven vessels meeting or crossing within half a mile, one short blast means "I intend to leave you on my port side," and the other vessel answers.
• Pushing ahead and towing alongside (Rule 24). Inland rules have their own light requirements for vessels pushing ahead and for barges being pushed, such as the special flashing light.
• Vessels constrained by their draft (Rule 28). This rule exists only in the International rules.

When you cross the line heading out, the rules change with you. Our Inland vs International guide lists the differences rule by rule.

Why it matters on the exam

Exam questions rarely ask you to name a specific line. They ask what Inland waters include, and the demarcation lines are part of the answer. Here are real questions from our bank, word for word:

Notice what the wrong answers have in common. "To the outermost aids to navigation" and "two miles offshore" sound reasonable, but the definition is about the demarcation lines, not buoys or distance from shore. Here's one more that tests why the line matters:

Quick ways to remember it

• Inside the line, Inland. Outside the line, International. That's straight from 80.01(b).
• Great Lakes, U.S. side, are Inland by definition, with no demarcation lines needed.
• Some whole regions are International, including Alaska's sounds, bays, harbors, and inlets, and Puget Sound and adjacent waters.
• When in doubt, check Part 80 and your chart.

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Frequently asked questions

What is a COLREGS demarcation line?

It's a line, set in 33 CFR Part 80, that divides waters where the International Rules (72 COLREGS) apply from waters where the U.S. Inland Navigation Rules apply. Waters inside the lines are Inland Rules waters, and waters outside are COLREGS waters.

Where can I find the COLREGS demarcation lines?

In 33 CFR Part 80, free on the eCFR, and printed on nautical charts as a dashed line labeled COLREGS Demarcation Line.

Are there demarcation lines on the Great Lakes?

No. Inland Rule 3 defines Inland Waters to include the waters of the Great Lakes on the United States side of the International Boundary, so they're Inland waters by definition.

Do the Inland Rules apply in Alaska?

33 CFR 80.1705 says the 72 COLREGS apply on all the sounds, bays, harbors, and inlets of Alaska.

Keep studying

• Rule 1: Application
• Inland vs International navigation rules
• What is COLREGS?
• USCG Rules of the Road test guide
• Free web study tool with 1,272 questions