The sand on beaches comes mainly from rivers, which carry sediment into the sea, while the waves distribute it along the coast. However, this natural process is significantly limited by the presence of dams—from large to small—which, according to experts, are a major cause of the disappearance of sandy beaches.

As reported by the Spanish news agency Efe, river management is becoming increasingly important as a tool for ecosystem restoration. At the same time, it can also help preserve beaches, particularly in areas under intense tourist pressure, such as the Mediterranean basin.

The Connection Between River and Sea

“The first thing we need to understand is that everything is connected: the river does not end at its mouth, but continues into the sea,” explains Alfredo Olero, a researcher and professor in the Department of Geography and Spatial Planning at the University of Zaragoza.

The sand on the beaches comes mainly from rivers, which transport the sediments, while the sea distributes them along the coastline through the waves.

He points out that dams cause a massive sediment deficit, as the amounts now reaching the sea are five to ten times smaller than they were before the dams were built.

More dams, fewer beaches

According to experts, there is a direct link between dams and beach loss.

The sand that forms beaches consists mainly of sediments transported from river basins. When this supply is cut off, beaches recede or even disappear, explains Askoa Ibizate, a professor of Physical Geography at the University of the Basque Country.

“The real problem isn’t that the sea washes away the sand, but that new sediment never reaches the coast,” he emphasizes, noting that much of it gets trapped in the dams that disrupt the natural flow of rivers.

In most cases, beaches are replenished with sand dredged from the seabed, where it ends up after severe storms—a process that entails high economic costs and significant environmental impacts.

At the same time, reduced sediment transport causes the loss of ecosystems and biodiversity, alters river morphology, and increases water velocity, which can even lead to the undermining of bridge foundations and other infrastructure.

The Impacts of Dams

Dams are among the most significant human interventions that restrict sediment transport, explains Carles Ferrer, a researcher and professor at the Polytechnic University of Catalonia.

Large dams trap nearly 100% of the sand and gravel, while smaller ones, used mainly for irrigation or hydroelectric power generation, reduce the river’s ability to transport sediment to the estuary.

Although they operate differently, both categories contribute to a reduction in the natural supply of sediment to the coast.

The Example of the Ebro Delta

A prime example is the Ebro Delta in Spain.

The Riparoya and Mecinenza dams, built in the 1960s near the river’s mouth, retain more than 90% of the sediment produced by the watershed.

Even smaller dams scattered along the river affect its dynamics, as they divert or retain part of the flow, thereby reducing the ability to transport sediment to the sea.

“If a river is the main source of sediment for a coastline, then dams inevitably lead to severe erosion, since the sea continues its erosive work as usual,” Ferrer emphasizes.

A similar situation occurred in the Lobrega Delta, where, following industrialization and the construction of numerous dams in the mid-19th and early 20th centuries, the shoreline began to gradually recede.

From the 1960s onward, the situation worsened further due to the construction of large reservoirs, changes in land use, and other human interventions that further restricted sediment supply.

Reservoir Draining

Experts point out that the accumulation of sediment in reservoirs gradually reduces their storage capacity.

For this reason, they believe that the downstream outlets of the dams should be opened periodically to remove the accumulated sediment and allow the natural supply of sediment to the coast to continue.

Although all large reservoirs have such infrastructure, in practice it is rarely used, since, according to experts, there is still a lack of comprehensive and environmentally oriented dam management.

Source: KYPE