Estimates of many deltas’ stabilities in the face of sea level rise were overoptimistic.
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Millions of people live on river deltas, occupying land that exists in the delicate balance between a river’s push and the ocean’s pull. Deltas are inherently transient, but according to a new study, many may be even more precarious than once thought, with unexpectedly high levels of land loss threatening to submerge these low-lying landscapes.
The insight that many deltas are degrading even faster than previously estimated stems from a new analysis of sediment flow rates led by Austin Chadwick, a geomorphologist at the California Institute of Technology.
When a river reaches the delta, its flow slows, and it begins dropping the sediment it carries, bolstering the land in the process. To date, geologists working to predict the future of a particular delta commonly compare the pace of sea level rise to the rate of sediment deposition. Their models assume sediment is spread evenly across the delta. But that’s not a realistic scenario, says Chadwick.
“The previous work has been very much a best-case scenario,” he says. “A river actually can’t be everywhere at once, and it can’t fight back against sea level rise everywhere.”
In reality, a river only builds new land where it’s currently flowing. Sediment deposited out of the water forms wide sandy pancakes called lobes. When too much sediment accumulates and blocks the river’s path, the river will jump course and start building a new lobe elsewhere. The old lobe, cut off from the sediment supply, will start to erode.
Chadwick and his team wanted to quantify how much land is gained and lost during these jumps and estimate how much future engineering projects can mitigate this land loss. They modeled a delta system using a new equation that factors in the interplay between sea level rise and river lobes’ unequal growth. They then confirmed the model using an artificial miniature delta in their lab before applying their findings to actual river systems.
The researchers’ prognosis for many deltas is bleak. The scientists estimate that given the pace of sea level rise, the Mississippi River, for instance, needs to deliver three times more sediment to the delta than previous estimates suggest to maintain the current area of dry land. Romania’s Danube River delta has it worse — it needs 10 times its current supply of sediment. In most cases, that’s more grit than the river can supply, suggesting these deltas are doomed to drown.
The new forecasting method, says Chadwick, “gives you a more realistic, but also a little bit more pressing, prediction.”
Not all deltas are in danger, though. China’s gritty Yellow River is so thick with sediment that it jumps course and forms a new lobe every decade. Engineers have even been able to corral the river with diversions to focus the firehose of sediment and bolster land in select locations. These sorts of projects come at a steep engineering and ecosystem cost, but they maximize the river’s land-building potential. Without diversions, a river may waste that much-needed sediment by pipelining it into the open ocean or building a lobe and abandoning it. For regions that can afford it, diversions offer a path to ensuring there is persistently dry and habitable land in choice delta regions.
Very few rivers, however, have the massive sediment supply of the super-silty Yellow River. The Mississippi River, in comparison, naturally switches course every 2,000 years. Louisiana’s Coastal Master Plan recommends diversions in some locations and river dredging elsewhere to build marshes and stabilize banks.
The clock is ticking for the mouth of the Mississippi and many other deltas. As the ocean rises, engineers will need even more sediment to fill the deeper water, says Ehab Meselhe, a civil engineering professor at Tulane University in Louisiana. A restoration solution that works today may not work 10 years from now. “The sooner we implement projects, the more likely they are to succeed,” Meselhe explains. “Time,” he adds, “is not on our side.”