Lago Doctrine

The Lago Doctrine is a relatively obscure Argentine legal-diplomatic position concerning the delimitation of binational lakes, particularly those shared with Chile along the Andean frontier. It holds that, absent a specific treaty provision, sovereignty over a shared lake should be apportioned by a median line equidistant from the respective shores, rather than by extending land borders straight across the water or by applying watershed (divortium aquarum) logic.

The doctrine emerged in the context of long-running Argentine–Chilean boundary disputes in Patagonia, where dozens of glacial lakes straddle the cordillera. Argentina has historically preferred coastline-based or median-line solutions in such waters, while Chile has at various points invoked the continental watershed criterion established in the 1881 Boundary Treaty and clarified in the 1902 King Edward VII arbitration.

In practice, the median-line approach has been used in several segments of the Argentina–Chile frontier, and similar logic appears in delimitations of Lake Titicaca (Peru–Bolivia) and the African Great Lakes, although those settlements rest on their own treaty bases rather than on Argentine doctrine as such.

Key features delegates should remember:

• It is a unilateral doctrinal position, not a codified rule of general international law.
• It interacts with, but is distinct from, the thalweg principle (which applies to rivers, tracing the deepest navigable channel).
• It is typically invoked alongside the uti possidetis juris principle when colonial-era maps are ambiguous about lacustrine boundaries.
• Application is usually negotiated bilaterally; the International Court of Justice has not endorsed a general "median line for lakes" rule, though equidistance is a common equitable starting point.

For MUN committees on territorial disputes, transboundary water governance, or Latin American regional integration, the Lago Doctrine illustrates how regional state practice can generate doctrinal labels even where universal codification is absent. Delegates citing it should be careful to distinguish Argentine doctrinal usage from broader customary international law on lake boundaries, which remains largely treaty-based and case-specific.