Why freshwater management matters for rising seas

We usually picture sea level rise through shrinking shorelines, flooded harbours and islands losing ground. But much of the damage also reaches wells, farmland and freshwater wetlands before land is permanently inundated. How we allocate river flows, manage groundwater and maintain sediment delivery can reduce that vulnerability. Freshwater management belongs at the centre of coastal adaptation.

As leaders gather in New York for the UN High-Level Meeting on sea level rise, the agreed draft declaration provides an opportunity to connect coastal resilience with the water decisions made both upstream and underground.

Salt in the wrong places
 

The IPCC projects that sea levels will continue rising for centuries to millennia. Its Sixth Assessment Report gives likely global mean increases by 2100 of 0.28–0.55 metres under very low emissions and 0.63–1.01 metres under very high emissions, relative to 1995–2014. Emissions reductions remain critical to limiting the scale of change and preserving adaptation options.

For freshwater, even modest rises matter. As the sea pushes inland, saltwater moves into coastal aquifers and up rivers and estuaries. It contaminates drinking water, degrades farmland and stresses freshwater-dependent ecosystems. For many small islands, a thin lens of fresh groundwater floating on seawater is the main water supply, and it is highly exposed. In the world's great river deltas, rising seas combine with land subsidence, groundwater over-extraction and the loss of river sediment trapped behind dams. There, the land is sinking while the sea is rising.

Coastal communities feel these effects long before permanent inundation arrives. Crops fail, wells turn brackish, and the risk of waterborne disease grows. Communities that are rich in culture but poor in options lose livelihoods, and people are eventually forced to move.

Building on the declaration
 

The new UN Declaration on Sea Level Rise recognises this reality more clearly than earlier drafts did, and IUCN welcomes that. The declaration explicitly recognises salinisation of arable land, surface water and groundwater. Its attention to science, adaptation finance and ecosystem-based approaches creates useful entry points for freshwater action.  It calls for sea level rise projections to be built into the planning, design and maintenance of critical infrastructure, including water systems. It also affirms the role of integrated coastal zone management and marine spatial planning. Now implementation needs to connect those commitments to water allocation, aquifer management and biodiversity protection.

Nature-based solutions have an important role, provided their benefits are clearly defined. Mangroves can reduce wave energy and erosion; wetlands can retain sediment where conditions allow. Protecting recharge areas and maintaining appropriate freshwater flows serve different functions. Coastal restoration should therefore demonstrate the water-security benefit it claims, rather than assume that every intervention prevents salinisation.

Freshwater biodiversity also needs explicit attention. As salinity increases, freshwater habitats can become brackish, changing the species they support. Conservation should identify freshwater refuges, maintain ecological connectivity and provide space for habitats to adjust where feasible. Total wetland area alone cannot show whether freshwater biodiversity is being maintained.

Managing the connections from source to sea

 

River discharge helps constrain saltwater intrusion, while sediment transport supports delta formation and maintenance. Reservoir operations, abstraction and sediment management therefore influence coastal resilience. During drought, maintaining downstream flows can involve difficult trade-offs with upstream irrigation, energy production and public an industrial water supply. Those choices require legitimate institutions, shared evidence and agreed priorities.

IUCN’s UNGA81 priorities recognise the need to connect water governance across freshwater, coastal and marine systems. Operationally, this means aligning adaptation plans, biodiversity strategies, wetland management and basin plans through shared objectives, responsibilities and budgets.  IUCN has urged Member States to strengthen coordination across the full set of relevant agreements: the Ramsar Convention on Wetlands, the Convention on Biological Diversity and the BBNJ Agreement all sit alongside UNCLOS and the Paris Agreement, and together they can address both the drivers and the impacts of sea level rise.  Transboundary basins show why this matters. IUCN's work with Lower Mekong countries on shared water resources, ecosystems and biodiversity reflects a simple lesson: a delta's future depends on decisions made across borders and far inland.

Four priorities for implementation
 

First, connect monitoring to decisions. Track groundwater salinity and levels, river discharge, sediment transport and wetland condition alongside sea level. Water authorities and utilities should agree thresholds that trigger action, such as adjusting abstraction, changing intake operations or activating alternative supplies. Monitoring slow-onset salinisation needs a clear route to an operational response.

Second, finance specific freshwater and biodiversity outcomes. Protect recharge areas, freshwater wetlands and river–floodplain connections, and restore coastal ecosystems where their benefits are demonstrated. Appraisal should assess salinity, habitat condition and reliable water access, alongside flood protection. Indigenous Peoples and local communities need meaningful decision-making roles, secure rights and accessible financing arrangements.

Third, plan across institutional boundaries. Basin organisations, groundwater agencies, utilities and coastal authorities should jointly assess future risks and coordinate investment. Plans need to recognise the limits of protection and provide for changing land use or infrastructure location where necessary. Nature-based and engineered measures should be assessed together for their long-term benefits, costs and ecological consequences.

Fourth, plan from Source to Sea. Incorporate downstream salinity needs and environmental flows into abstraction permits, reservoir operations and drought arrangements, based on local evidence. In shared basins, agree cooperation mechanisms and responsibilities for implementation. Governments should identify who funds the measures, who bears adjustment costs and how affected communities participate.

From New York to the UN Water Conference
 

The Declaration commits to mainstreaming sea level rise into all relevant processes and to convening a further high-level meeting before the end of the 84th session. The next test comes soon.  The UN Water Conference in December offers a practical opportunity to carry this agenda forward. Governments and financing institutions should make freshwater security and biodiversity explicit outcomes of coastal adaptation programmes, backed by named implementing institutions, budgets and measurable results.

IUCN’s proposed Global Framework for Water Governance and Action offers a means to strengthen coherence between commitments, implementation and finance. Sea level rise makes that connection tangible: the decisions governing river flows, aquifers and ecosystems already shape the options available to coastal communities. Our response needs to follow the water.