## Facts

- Costa Rica and Nicaragua share a boundary largely connected to the San Juan River; Nicaragua holds sovereignty over the river while Costa Rica holds specified navigation rights under an 1858 treaty and subsequent decisions.
- Costa Rica alleged Nicaragua entered and occupied a wetland area near the river’s Caribbean mouth, excavated three artificial channels (caños), conducted related dredging, and placed military personnel there, harming a protected wildlife refuge.
- Nicaragua alleged Costa Rica built Route 1856 along the river’s right bank close to the border, causing sediment and debris to enter the river and harm Nicaraguan territory.
- The disputes raised overlapping questions of territorial sovereignty, treaty-based river navigation, and transboundary environmental obligations, including impact assessment and prevention of harm.
- The court ordered provisional measures (2011; 2013) restricting personnel and works in the disputed territory and later joined the proceedings, issuing a single merits judgment in 2015.

## Issues

1. Which state had sovereignty over the disputed wetland/coastal area at Isla Portillos/Harbour Head Lagoon.
2. Whether Nicaragua’s excavation works and military presence violated Costa Rica’s territorial sovereignty and related international obligations.
3. Whether Nicaragua breached procedural environmental duties by failing to conduct an environmental impact assessment (EIA) and to engage Costa Rica before undertaking works posing a risk of transboundary harm.
4. Whether Nicaragua caused significant transboundary environmental harm to Costa Rica’s protected areas.
5. Whether Nicaragua unlawfully interfered with Costa Rica’s treaty-based navigation rights on the San Juan River.
6. Whether Costa Rica breached customary international law by failing to perform an EIA for construction of Route 1856.
7. Whether Costa Rica breached treaty duties (including under the Convention on Biological Diversity) and any duty to notify/consult Nicaragua regarding the road.
8. Whether Costa Rica’s road caused significant transboundary harm to Nicaragua.

## Decision

- The court held Costa Rica had sovereignty over the disputed territory where Nicaragua excavated the caños and stationed personnel (while recognizing Nicaragua’s sovereignty over a limited coastal strip/beach area).
- The court found Nicaragua violated Costa Rica’s sovereignty by excavating the caños and establishing a military presence in Costa Rican territory.
- The court found Nicaragua breached binding provisional measures by maintaining or sending personnel and continuing certain works contrary to the court’s orders.
- The court held Nicaragua was required to perform an EIA for works posing a risk of significant transboundary harm and failed to meet that requirement.
- On navigation, the court found Nicaragua breached Costa Rica’s treaty-based navigation rights in certain respects, while reaffirming Nicaragua’s sovereignty over the river subject to those rights.
- The court held Costa Rica violated customary international law by failing to conduct an EIA for Route 1856.
- The court rejected Nicaragua’s claims that Costa Rica violated Article 14 of the Convention on Biological Diversity in the manner alleged and rejected broader notification/consultation claims not established beyond the EIA duty.
- The court held Nicaragua did not prove the road caused significant transboundary harm; therefore, no compensation was ordered against Costa Rica.
- The court held Nicaragua owed compensation to Costa Rica for material damage from Nicaragua’s unlawful activities in Costa Rican territory, with quantum left for later determination.

## Legal Principles

- Territorial sovereignty is violated by unauthorized physical activities and official presence (including military personnel) carried out in another state’s territory.
- Provisional measures indicated by an international court are legally binding; noncompliance is an internationally wrongful act.
- Under customary international law, an EIA is required where a planned activity carries a risk of significant transboundary harm; the obligation is one of due diligence.
- The scope and content of an EIA depend on the nature and circumstances of the project; customary law does not impose a single fixed methodology.
- The substantive duty of due diligence not to cause significant transboundary harm requires proof of “significant” harm, which is more than detectable but need not reach the level of serious or substantial.
- Sovereignty over an international boundary river may coexist with the neighboring state’s treaty-based navigation rights; the sovereign may regulate navigation only to the extent consistent with those guaranteed rights.

## Conclusion

The court awarded Costa Rica key relief on sovereignty, provisional-measures compliance, and compensation for Nicaragua’s unlawful acts in Costa Rican territory, while also finding that both states breached customary EIA obligations; it rejected claimed road-related compensation because significant transboundary harm was not proven and clarified the balance between river sovereignty and treaty navigation rights.


Certain Activities Carried Out by Nicaragua in the Border Area (Costa Rica v. Nicaragua) & Construction of a Road in Costa Rica along the San Juan River (Nicaragua v. Costa Rica), 2015 I.C.J. 665 (Int'l Ct. Just. 2015)