A study led by Wageningen University & Research shows that human interventions have significantly changed tides in river estuaries over the past centuries. In many regions around the world, the difference between high and low tide has increased, and the tidal wave is moving inland faster. These changes often appear to have a greater impact than the effects of sea-level rise.
Reconstruction of more than a century of tides
For the study, published in Nature Geoscience, historical maps, tide tables and historical measurements were compared with modern data. PhD Candidate Joris Beemster spent years researching in libraries and archives around the world for this, studying data from the past 50 to 200 years. This enabled the reconstruction of the development of 25 estuaries (wide river mouths) in Europe, Asia and North America, from the coast to far inland.
"We saw that tides have changed significantly. The flood tide rises more quickly, is shorter and more powerful, and penetrates further inland."
- Joris Beemster
- PhD candidate
Notably, the greatest changes often occur far from the coast, sometimes more than 100 kilometres inland." Some estuaries also appear to be much more vulnerable than others. In some systems, the difference in height between low and high tide increased locally by over two metres - more than three times the historical level - while changes in other estuaries remained relatively limited.
Human influence is greater than previously thought
The changes are mainly the result of human interventions in estuaries, such as deepening shipping channels, land reclamation, and straightening river courses. "Such actions have changed the shape and depth of estuaries, reducing their natural slowing effect on the tide," Beemster explains. "This allows the tidal wave to travel faster and become stronger."
Relatively stronger impact further upstream
The study shows that local human interventions have so far often had a greater impact on water levels than sea-level rise. While sea levels are rising gradually, changes to estuaries have led to faster and larger shifts, especially inland. This means that water level observations at the coast do not always give a good picture of what is happening further upstream. In fact, water levels further inland can change to a larger extent than what would be expected from sea-level rise alone.
Opportunities for the future
Because humans drive these changes, they also hold the key to solutions. Beemster: "By restoring parts of estuaries to a more natural state, for example by making them shallower or restoring intertidal areas, the tide can be slowed down again. This is encouraging: if we could reverse some interventions, the impact of sea-level rise inland could also be reduced. In this way, estuaries could become more resilient again, while also benefiting nature and water quality."
