## Facts

- Lac Lanoux is located in France; its outflow feeds the River Carol, which runs into Spain where it supports existing uses, including the Canal of Puigcerdà.
- France planned a hydroelectric project to divert Lac Lanoux waters to the River Ariège (within France) for power generation and then return the waters to the upper Carol.
- The frontier-area water regime was governed principally by the Treaty of Bayonne (1866) and its Additional Act, including provisions addressing “waters of common use” and cross-border interests.
- Spain objected that the diversion would alter the natural basin, place effective control in French hands, and risk harm to Spanish downstream users and treaty-protected rights.
- Spain argued the Additional Act required prior bilateral agreement for works of “general convenience” affecting shared waters; France denied any Spanish veto and asserted it would return the waters and prevent injury to Spanish uses.
- After negotiations through diplomatic channels and commissions failed to yield agreement, the states submitted the dispute to an ad hoc arbitral tribunal to interpret the 1866 treaty instruments as applied to Lac Lanoux.

## Issues

1. Whether the 1866 Treaty of Bayonne and Additional Act required France to obtain Spain’s prior agreement before implementing the Lac Lanoux diversion project.
2. What obligations the treaty regime imposed on France to consider Spanish interests and protect Spanish rights in designing and implementing the project.
3. Whether international law permits an upstream state to proceed unilaterally with works on its territory affecting a shared watercourse where downstream rights are not impaired.

## Decision

- The tribunal rejected Spain’s claim that France needed Spain’s prior consent to carry out the project.
- The tribunal held France could proceed with the diversion on French territory, conditioned on respecting Spain’s treaty-based rights and interests.
- The tribunal found the treaty regime did not create joint control or a veto over French works; limitations on territorial authority could not be presumed without clear basis.
- The tribunal emphasized France’s duties to take Spanish interests into account and to negotiate in good faith, and it relied on the project’s safeguards, including full return of diverted waters to the Carol above the Spanish canal intake.

## Legal Principles

- Territorial authority over waters within a state’s territory is presumed; treaty-based limits on that authority must be shown and are not inferred by expansive interpretation.
- A co-riparian may have a duty of notification, consultation, and good-faith negotiation regarding works affecting shared waters, but that duty does not, without specific legal text, grant the other state a unilateral veto.
- International law does not bar a state from constructing facilities that could enable harm in the future; responsibility turns on breach of international obligations and actual impairment of protected rights.
- Compliance with a treaty regime concerning shared waters requires genuine consideration of the other state’s interests and appropriate safeguards to preserve established, protected uses.

## Conclusion

The tribunal concluded that France was not required to secure Spain’s prior agreement for the Lac Lanoux diversion, but France remained bound to negotiate in good faith and implement the project so that Spain’s treaty-protected downstream uses and interests were not injured, including by ensuring effective return of the diverted waters to the River Carol.


Spain v. France (Lac Lanoux Arbitration), 12 R.I.A.A. 281 (1957)