Lines In The Sea

Who says you can't draw a line in the water? Nations and private property owners have been doing it for hundreds of years, to define their property rights and defend their seaward borders. Today a whole series of overlapping and sometimes conflicting laws, decrees, and treaties exist that draw just such lines in the sea. Because you can't actually draw such a line, they are usually drawn with reference to something on land. In the United States, the following lines are drawn:

Mean higher high water. In areas with two high and low tides a day, this line marks the average of the higher of the two high tides over a period of several (usually 19) years. The State of Texas uses this line as the seaward boundary of private property; you may not build a fence below it, for instance.

Mean high water or shoreline. This is the average of all high waters over the 19-year period. Fourteen US states use this as the seaward property line; areas above this are called privately owned uplands.

Mean low water or coastline. The average of all low waters over the datum epoch period. Seven states use this as the private property line. Areas above it are called state owned tidelands or inland waters.

Mean lower low water. The average of each daily lowest of the two low tides. This is the baseline from which US territorial waters are measured seaward. Areas above this line are called state submerged lands.

Three-mile limit. This distance is the range of a shore-based cannon in the early 1700s, and thus served as a practical definition of a nation's seaward boundary. Although now largely obsolete, it still has some applications. Check out an online chart of southeast Alaska here and you can still see the "doughnut holes" created by the three-mile boundary.

Twelve-mile limit. This is the generally accepted seaward limit of a nation's laws, established by international convention. Waters landward of this line are called the territorial sea; waters seaward are called the high seas. Some nations claim an exclusive economic zone extending up to 200 miles off their baseline. Although marked on some nautical charts with a fish symbol, these zones include not only fishing but oil and natural gas extraction rights, as well as environmental law enforcement. The image above shows both the United Kingdom's twelve-mile limit and areas further offshore it has claimed.

Disputes in these boundaries still exist. The US refusal to recognize North Vietnam, and thus its twelve-mile limit, was cited (at the time) as a cause of the Gulf of Tonkin incident. Libya's attempt to claim the entire Gulf of Sidra as territorial waters led to conflict with the US twenty years later. Even today, both the US and Canada claim jurisdiction over potentially oil-rich areas of the Beaufort Sea.