Rivers are a natural boundaries and a pretty obvious way to determine who controls land on either bank. Well, not withstanding occasional riverbed shifts. However rivers are also natural resources in their own right. They provide drinking water, irrigation, food and transportation. Those who control territory abutting a river naturally want to own and control access to the precious water coursing past it.
Governments have devised ways to prevent or resolve disputes but there are only two basic approaches with minor variations. Either one party controls the entire river or both of them split it. This can accomplish by negotiation, tradition, logic, adjudication, luck, force or by any other reason. No matter how they do it, everything boils down to either sharing control or not.
One Side Owns All
The Potomac River boundary is an example of “one side owns all”. In this instance Maryland and the District of Columbia own the river. Virginia, their cross-border neighbor does not. Period.
This arrangement traces to a 1632 Royal Charter that continued across the centuries. Thus, any map clearly shows the relationship (here, for example). The border extends all the way to the Virginia shoreline.
So it sounds like such an elegantly simple solution. Unfortunately, water rarely remains constant for long. Even in what would seemingly be a cut-and-dried instance, governments need to form agreements to use certain points such as the low-tide mark, the crossing of tributaries flowing into the river, and landform changes taking place during cycles of drought and flood. Disputes continue to flare into the modern era as water becomes an increasingly valuable commodity.
Sharing Ownership
Actually it’s more common for neighbors to share ownership. That’s the case of the Columbia River between Oregon and Washington (map).
Grotian Method
So an early method involved an equal split right down the middle based on fundamentals of natural law advocated by Hugo Grotius in the early 17th Century. Such Grotian theories formed the underpinning of international law and provided a rational basis for dispute resolution and economic efficiency.
However as in the Columbia River example — and indeed for most river boundaries — surveyors discarded the Grotian model. Therefore, as a result, some of the Columbia River islands fall within Oregon and others in Washington. So the border seems to zigzag between the Columbia’s riverbanks.
Thalweg
Thus borders in the modern era generally follow the thalweg.
“For navigable rivers, which are commonly used for demarcating international boundaries, the legal rule dictates that the boundary line follow the middle of the thalweg, or the navigable channel through which the current flows downstream. If the river is not navigable, the older, Groation rule applies in which the boundary runs down the middle of the stream.”
Joyner, Christopher C.; International Law in the 21st Century: Rules for Global Governance. Lanham MD: Rowman & Littlefield, 2005
So the thalweg is the midpoint of the deepest contiguous line along the bottom of a riverbed. Also it is generally its point of greatest flow and natural direction.
The derivation involves two German words Tal meaning valley, and Weg, meaning way. Alternately some call it the “valley line”. Since rivers twist and turn, and currents drop sediment in some locations and erode them in others, it’s no wonder that the thalweg usually swings back and forth between riverbanks.
Got all that? This will be useful for future posts.
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