Human activities have changed the shapes of river deltas

A model has been devised that quantitatively describes how the shape of a river delta is affected by sediments, tides and waves. It reveals that the area of delta land is increasing globally, as a result of human activities upstream.

Because of their rich soils and convenient positions for trade and transport, many deltas have also become hotspots of socio-economic development. The Nile delta, for example, with its iconic triangular shape, has been one such locus for more than 5,000 years. 

Nienhuis and colleagues used their model to estimate the effects of upstream human interventions on delta morphology during the period 1985–2015. They found that dam building led to decreases in sediment delivery, whereas accelerated soil erosion caused by deforestation increased sediment delivery. Of the approximately 11,000 deltas analysed, about 9% are significantly affected by reduced sediment delivery, producing a total land loss of 127 square kilometres per year, whereas about 14% received increased sediment, causing a total gain of 181 km2 yr–1 during the study period. The reason more deltas have experienced an increase in sediment delivery, rather than a decrease, is simply that the effects of massive deforestation have outpaced sediment trapping by dams.

Large deltas, such as those of the Niger, Huang He and Mekong, have great socio-economic value. Such densely inhabited deltas typically experience many pressures in addition to changes in sediment delivery, such as stresses associated with groundwater pumping, sand mining, dyke construction and loss of biodiversity.

By accounting for the baseline effects on deltas of human activities such as dam building and deforestation, Nienhuis and colleagues have provided a fundamental framework that will help assessments of the impacts of climate change for decades to come.

Nick van de Giesen from the Department of Water Management, Delft University of Technology, 2628 Delft, the Netherlands is the writer of this article published in Nature 577, 473-474 (2020).

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