Estuaries around the world expected to become saltier in the coming decades

In estuaries—the transitional zones between rivers and the sea—fresh and salt water are constantly battling for dominance. But due to climate change, the saltwater is gaining ground. New research by Utrecht University’s Institute for Marine and Atmospheric Research (IMAU) in collaboration with Deltares shows that saltwater intrusion—where seawater pushes inland into rivers—is on the rise globally. By analysing eighteen estuaries across the globe, researchers found that in 89% of cases, the salt front is moving further upstream, primarily due to sea level rise and reduced river discharge, especially during the summer months. Regions experiencing drought and low river flows are expected to become increasingly salty over the coming decades.

As far back as the 10th century, Dutch farmers were draining their land—pumping out water to lower the groundwater level and make the soggy landscape suitable for agriculture. As the land began to subside, they built dykes to keep the sea at bay and preserve freshwater for farming and drinking.

But those conditions are changing, warns physical oceanographer Huib de Swart. “During dry periods, when rivers carry less water, saltwater can push much farther inland.” And this is not just a local issue: saltwater intrusion threatens freshwater availability in coastal regions across the world—a problem that is expected to intensify with climate change.

Rising salt levels

In this study, researchers examined changes in saltwater intrusion for eighteen river systems worldwide. Using climate simulations, they projected future river discharges and relative sea level rise, allowing them to forecast saltwater intrusion trends through the end of this century.

“This is the first study to assess the combined effects of changing river discharge and sea level rise on saltwater intrusion at a global scale,” says climate physicist and project lead Henk Dijkstra. Along with colleague Huib de Swart, he supervised lead author Jiyong Lee during his postdoctoral research.

River discharge and sea level rise

Over the coming decades, decreasing river discharge poses the greatest climate-related risk to freshwater supplies. However, the study finds that by the end of this century, the impact of sea level rise on saltwater intrusion could be roughly twice as great as that of reduced river flow. The likelihood of extreme saltwater intrusion events—currently considered “once-in-a-century” phenomena—may increase by as much as 25%.

This is the first global study to map how sea level rise and reduced river discharge jointly affect saltwater intrusion

Rhine-Meuse Delta

The Netherlands is no exception when it comes to the growing impact of saltwater intrusion. Saltwater is expected to reach further upstream in Dutch rivers, while seepage of saline groundwater under dykes may also increase, leading to saltier soils. This could have significant consequences for agriculture and ecosystems, especially in the western part of the country. Certain crops may struggle, and some forest areas could have difficulty surviving in saltier conditions.

De Swart emphasises that the Netherlands must remain vigilant. Even regions that currently experience little to no saltwater intrusion may face problems in the future. “Right now, saltwater can reach up to 35 kilometers inland during extreme events. Once every ten years, it may push up to 40 kilometers. But our model projections show that this could become much more common—potentially every year,” he says. “In a hundred years, the average saltwater intrusion may extend 10 to 15 kilometers further inland than it does today.”

Adapting to change

According to De Swart, adaptation will be essential. Drinking water systems will need to be managed more carefully—for instance, by storing water during wet months and reusing greywater, such as rainwater, for flushing toilets or irrigating gardens. Agriculture may need to switch to salt-tolerant crops. And water-intensive industries like paper manufacturing will need to reassess whether their current practices remain viable. “We’ll have to get used to the idea that unlimited access to fresh water is no longer a given,” De Swart concludes.

We need to learn that we can't keep using freshwater without limits