Restoration of the Oka River’s upper estuary, part of the Urdaibai Biosphere Reserve, Spain

The upper estuary of the Oka River, within the Urdaibai Biosphere Reserve on Spain's Basque coast, has been reshaped by centuries of agriculture, cattle-raising and shipyard activity, including a canal that reduced the estuary's natural ecosystem function. Climate change is projected to increase river flow, flooding and erosion risk and to raise sea levels by 29-49 cm by 2100 in the Basque Country, compounding the estuary's already-limited capacity to adapt naturally because of dykes and other artificial barriers.

The restoration aims to improve the estuarine ecosystem's resilience to climate impacts such as sea-level rise and changing water regimes, increase local biodiversity by recovering ecosystem function, and increase sustainable public use of the Urdaibai reserve.

Work included permanently flooding the Barrutibaso area to recover former marshland, restoring part of the original intertidal channel in the lower estuary, removing dykes and other artificial barriers, and building embankments to create lagoons intended to become future intertidal zones. A 14 km pedestrian and cycling path network with a bridge was built to connect neighbouring municipalities to the estuary, alongside environmental-education tools including a smartphone app and on-site information panels, and eradication of invasive species (Baccharis halimifolia and Cortaderia selloana) across about 700 hectares.

The project was fully funded by the Department of Environment, Land Planning and Housing of the Basque Government, with a total budget of about EUR 2.5 million. Restored marshland is expected to provide new habitat for amphibians and birds and to help displace some invasive species through the reintroduction of permanent brackish water, though no quantified biodiversity outcome data is reported.

The new path network is reported to have improved sustainable access and raised public awareness of the restoration, but the project has been only partially implemented: some municipalities in the area did not participate, and Spain's 1998 Coastal Law gives landowners within 100 metres of the river the right to object to planned works, which has delayed removal of some dams. Negotiations to complete the remaining phases were ongoing at the time of reporting.