River deltas are changing shape

Deforestation and damming have altered the shape of river deltas across the globe, contributing to a net 54 square kilometres of land gained per year over the past three decades, according to a study published in the journal Nature.

These landforms are home to hundreds of millions of people, says lead author Jaap Nienhuis, from Utrecht University in The Netherlands. “Think of Bangladesh, Vietnam, Netherlands, New Orleans.” 

Throughout human history, deltas have been hubs of trade, transport, farmland and settlement, driving socio-economic development. They are also important environmentally, as they harbour diverse ecosystems.

Deltas are formed by sediments deposited as rivers enter still or slow-moving water, such as oceans, estuaries and lakes, but their sediment supply is affected by waves, tides and rivers, as well as damming and deforestation.

Nienhuis and colleagues developed a model to predict how delta shapes are affected as these factors influence sedimentation.

“For example, if we dam a river that stops river sediments from nourishing a delta, we want to know where and how fast coastal erosion can take place,” he explains.

Developing mathematical theories, they used global models of waves and tides to apply the theories to nearly 11,000 deltas, including everything from small bayheads to mega-deltas, then validated them using satellite imagery.

Results showed that waves cause more delta land loss than tides, and that the impact of human activities on sediment flux explained 16% of changes to delta land areas.

“We found that deltas globally are gaining land, in part because of deforestation that causes land degradation and led to extra sediments supplied to each delta,” says Nienhuis.

But river damming has severely reduced sediment flux, causing 23 square kilometres of delta land area to be lost.

Despite the net land gain causing river deltas to grow, this is highly likely to change with sea level rise and continued dam construction, says Nienhuis.

“Often times dam-building occurs without thinking too much about the consequences for downstream areas. Our findings can help tell people how much land loss would be expected for certain types of dams, for example.”

There was significant variation among the continents.

The largest delta changes were in south, southeast and east Asia, contributing to 57% of new land gained and 61% of land loss overall. Deforestation outstripped damming in this region, leading to the largest amount of land gained.

North America was the only continent with a net decrease of delta land area, which the authors attribute largely to the fast decline of the Mississippi Delta, partly because of damming, while arctic river deltas had virtually no change.

The authors acknowledge the limitations of their global-scale model, which doesn’t consider all human-delta interfaces.

As Nick van de Giesen from Delft University of Technology notes in a related commentary in the journal, its comprehensive nature comes at a cost to fine-level precision, which will likely produce some inaccuracies in the predicted shapes of some deltas.

Population-dense areas with large deltas will, therefore, need local attention to factor in additional pressures such as groundwater pumping, sand mining, dyke construction and loss of biodiversity.

“Nonetheless,” van de Giesen writes, “the model’s results are statistically valid at a global level,” and it fills in some important gaps.

“Deltas connect the terrestrial and maritime branches of the hydrological cycle and the associated sediment fluxes. As such, they encapsulate many key indicators of global change.”