Editor's note: The following article is drawn from a long-form analysis of the dynamics of river deltas in the Bulletin's March/April 2009 edition. That analysis can be found here.
The world's coastlines are highly dynamic landforms, tapestries of terrestrial and aquatic life and distinctive ecosystems. The coastal zone also contributes to human sustenance, culture, and economics in ways that belie its small size, but 5 percent of Earth's landmass. Three-quarters of global population lives within the coastal zone, and more than half of global gross domestic product is generated within it.
Within the coastal zone, river deltas are among the most consequential of landforms and have played an essential role in human history, serving as cradles of civilization, testing grounds for early agriculture, and the birthplace to hydraulic engineering. Today, deltas are home to a half-billion or more people. More than 200 million people are crowded into the Ganges, Nile, and Mekong deltas alone, and many of Asia's current or emerging megacities are located within deltas.
As low-lying and low-relief plains, deltas are highly sensitive to even small changes in sea level and thus rightfully claim ground in the greenhouse warming debate. But the fragility of deltas is not solely a consequence of rising ocean waters. Coastal deltas are the by-product of a remarkable biogeophysical balancing act.1 In a typical year, a river furnishes supplies of fresh water, mineral sediments, inorganic nutrients, and organic materials from the upland drainage basin. Muddy floodwaters rise and spread over deltaic wetlands from main and so-called distributary channels, transporting mass and energy laterally across the delta, in essence nourishing the landscape. Wetland vegetation helps to stabilize the benefits of these inputs. The interplay of these factors regularly reconfigures deltas, often abruptly and often catastrophically, on both geologic and historical time frames.2 This characteristic pattern of change within deltas, however, has rarely forced present-day natural systems over the threshold to collapse. But this might soon change.
The human desire for stability forms a basic tension with the dynamism by which natural deltas maintain themselves. Humans have devised an array of stabilizing strategies, enforced by hydraulic engineering, to create what is essentially a custom-made coastal habitat for human beings and their property. Human activity nearly always tips the scale in favor of forces bearing a destructive effect on deltas. Reservoirs upstream trap riverborne sediment before it can get to the coasts; irrigation depletes river flows; and groundwater and hydrocarbon extraction cause sediments to subside. Local observations support this assertion and highlight the many interconnected and costly impacts on a delta losing ground. Spain's Ebro River delta has retreated more than 10 meters per year during the last 40 years, and to stave off the growing coastal erosion problem, the government transports thousands of truckloads of sediment into the coastal zone each year. The Chao Phraya delta on which Bangkok rests experiences a net elevation loss of several centimeters per year, a decrease more than 10 times greater than rates of global sea-level rise. In the Ebro, Po, and Nile deltas, humans have converted nearly all wetland habitats to agriculture, and more than half of the wetlands in the Rhone River delta in the south of France have been reclaimed. These are but a few of the many examples we see of coastal deltas in a state of inadvertently engineered disequilibrium.
While their story is predominantly about increased risk, modern deltas are highly dynamic and malleable. Under the appropriate conditions, their deterioration can be slowed and even reversed. To do so requires an understanding of their basic building blocks--the strength of particulate matter sources, water inflows and their potential to overtop banks, sedimentary environment, the presence or absence of stabilizing vegetation, and ocean-side dynamics. Scientists have made great progress in observing deltas in nature and then creating new ones that bear an amazing likeness to real systems, either through controlled experiments (such as the aptly-named Jurassic Tank or sophisticated computer simulations. An improved understanding of deltas' functioning is an essential first step to instituting strategies to resuscitate them.
Reestablishing river-borne sediment and water sources is at the heart of any such intervention, but the reestablishment of ecosystems and well-adapted plant communities is also crucial. This approach is currently under consideration in the Wax Lake Delta, an active part of the larger Mississippi Delta system. Following massive flooding in 1973, river discharge was redistributed into the Atchafalaya River. As a consequence, the Wax Lake Delta today receives from 15 to 30 percent of the Mississippi River's sediment and has built more than 40 square kilometers of new land. It represents a natural experiment in how deltas grow and provides important clues as to how such systems can be rehabilitated using the building blocks of water flows, sediment fluxes, and natural biotic factors, which help to stabilize the system. Other novel engineering approaches include the pressurized injection of water and sediment directly into a delta to restore lost elevation, an approach scientists are exploring to alleviate the inundation of Venice, Italy.
At the simplest level, governments and the public need to fully appreciate that the state of deltas represents the legacy of natural history, human decisions, and ongoing sea-level rise, and that their actions in response to current conditions will shape delta vulnerabilities well into the future. The collective significance of transformations of so basic a set of building blocks of the Earth system, namely the water and sediment that create deltas, should give all humans pause as we assess our prospects for sustainability moving forward into the 21st century.
With Earth's population reaching perhaps 10 billion people by mid-century, humans will need to juggle the increasingly complex and tightly linked strategic imperatives of food and energy security, economic development, and carbon mitigation. Our stewardship of the world's deltas, an arguably less complicated challenge, offers opportunities to demonstrate our willingness and technical capacity to confront still larger global environmental threats.
1George Postma, "Depositional Architecture and Facies of River and Fan Deltas: A Synthesis," in A. Colella and D. B. Prior, eds., Coarse Grained Deltas, special publication of the International Association of Sedimentologists, pp. 13–8.
2George Postma, "Physical Climate Signatures in Shallow- and Deep-Water Deltas," Global and Planetary Change, vol. 28, no. 1-4, pp. 93–106.; James P. M. Syvitski et al., Nature Geosciences (in review).
