Rivers are a natural boundaries and a pretty obvious way to determine who controls land on either bank, 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. Methods have been devised to prevent or resolve disputes but there are only two basic approaches with minor variations. Either one landowner will control the river entirely or residents on opposing banks will find a way to split it. This can be established by negotiation, tradition, logic, adjudication, luck, force or by any other reason, but it all boils down to either sharing control or not.
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“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. 43.
So the thalweg is the midpoint of the deepest contiguous line along the bottom of a riverbed, and is also generally its point of greatest flow and natural direction. It is derived from the German words Tal meaning valley, and Weg, meaning way, and is alternately called 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.