rule of the thalweg

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[from German: literally, valley line]

The rule for determining the boundary line between two states that are separated by a navigable river containing a newly formed island. According to this rule, the boundary line moves with the centre of the navigable channel, i.e. it is delineated as being the centre of the course with the strongest current, so that the newly formed island must lie on one side of it or the other. In the case of Iowa v Illinois 147 US 7 (1893) the expression “middle of the Mississippi River” was interpreted as the “middle of the main channel” or “thread of the stream”. On non-navigable rivers, however, the middle line of the river will mark the boundary between the two states between which it flows. Thus, a newly formed island might well fall partly on one side of the boundary line and partly on the other. See also accretion; avulsion.

Subjects: Law.

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