DATE: September 27, 2019
SNIP: River deltas—like those at the mouth of the Mississippi, Nile, or Ganges—barely rise above sea level. Among the regions most imperiled by climate change, they barely rise to the level of public attention.
That’s unfortunate, because deltas—which form where large rivers deposit sediment as they flow into the ocean—are home to half a billion people. They also support some of the planet’s most productive agricultural regions and fish harvests.
Our warming planet poses an existential threat to deltas, a reality made clear in the Special Report on the Ocean and Cryosphere in a Changing Climate, released this week from the Intergovernmental Panel on Climate Change (IPCC).
The IPCC report states with “high confidence” that deltas will face “high to very high risks” in the future from rising sea levels, even under scenarios where the world rapidly reduces emissions of greenhouse gases and minimizes the rise in global temperatures and the subsequent melting of ice sheets and glaciers.
But the report also identifies other threats to deltas, including the loss of the sediment needed to replenish them and keep them above the rising seas. And while climate change requires a global solution, sediment loss has solutions that are far more local and arguably more tractable in the short term.
A river is brown because it is more than just a flow of water moving downstream, it is also a flow of sediment. These sediments, including silt and sand, are the products of erosion across a river’s basin (all the land that eventually drains into the river).
When a river reaches the ocean, it loses its ability to continue carrying its “sediment load” and most of that silt and sand is deposited. Over long periods of time, this process results in the creation of new land, often in the general shape of a triangle.
A sizable fraction—nearly half—of that sediment [now] gets “captured” along the way in the reservoirs on the Missouri River and other major tributaries of the Mississippi. A large dam backs up a reservoir which greatly slows or halts a river’s flow, causing its sediment to drop out. These “reservoir deltas” expand at the expense of the river delta downstream. The loss of sediment supply robs a delta of the raw material it needs to keep pace with a range of forces intent on tearing it down, including rising sea levels and waves and storms that erode land.
While the Mississippi River Delta has lost about half of its sediment supply due to upstream reservoirs, many river deltas have lost nearly all their sediment. For example, the deltas of both the Colorado River and the Nile River have lost more than 98% of their sediment supply.
With 50,000 major dams worldwide, people now have a considerable impact on the global supply of sediment for deltas. Reservoirs trap approximately one quarter of the global annual flux of sediment—silt and sand that would otherwise reach deltas and the ocean. And the problem is only going to worsen: thousands more dams, mostly for hydropower, are under development or proposed for many of the world’s rivers that still supply much or most of their original sediment loads to their deltas.