Another post about river songs? Not this one. Because I’ve remembered that I don’t work for Rolling Stone and was actually hired by the Conservancy as a river scientist, this post is about coastal deltas, those places where big rivers meet the sea.
When a river flows out into a large body of water, like the ocean, the sediment it has been carrying — such as sand and silt — succumbs to gravity and falls downward. Over long periods of time, this process results in the creation of new land, often in the basic shape of a triangle (hence the name "delta" after the Greek letter Δ; see photo above of the inverted green triangle of the Nile River delta).
Imagine a particle of soil eroding from a Nebraska hillside, being carried away by the “Big Muddy” (the Missouri River), then down the Mississippi, and finally settling out to help build up the marshland of coastal Louisiana. Over millenia, this process has constructed deltas that collectively are home to 500 million people — or approximately 1 out of every 12 people on Earth — and some of the most important agricultural areas on the planet. Deltas are also incredibly important for biodiversity and as nursery areas for fish and shellfish.
But in much of the world, the processes that build and maintain deltas have been greatly diminished or even halted, as described by James Syvitski and co-authors in a paper last fall in Nature Geoscience. Worse, rising sea levels threaten to erode and flood delta lands, and salinate (make salty) deltas’ groundwater, wetlands and river channels.
For a paper in a scientific journal, Syvitski’s has a very concise and accessible title: “Sinking deltas due to human activities.”
What are these human activities? One is the construction of “armored” levees around delta river channels to lock the river in place and prevent flooding. But keeping floodwaters away from a delta’s land robs it of the very sediment needed to maintain and continue building the delta.
Dams are a second major cause of sinking deltas. A large dam creates a reservoir behind it which greatly slows or halts a river’s flow, causing its sediment to drop out. Behind most big dams, a delta forms where the river enters the dam’s reservoir. These “reservoir deltas” grow even as the downstream “original deltas” shrink.
With 50,000 major dams worldwide, and thousands more being built and planned, people now have a major effect on the global movement of sediment. In an earlier paper, Syvitski and co-authors estimated that reservoirs trap approximately one-quarter of the global annual flux of sediment — sediment that would otherwise reach deltas and the ocean — and that, cumulatively, more than 100 billion metric tons of sediment has been captured behind dams.
Syvistski identifies a number of deltas that are in “greater peril” and many of these are deltas downstream of major dams that capture nearly all the riverborne sediment. For example, the Colorado Delta’s annual sediment deposits have declined by 100 percent and the Nile Delta’s sediment inputs have dropped by 98 percent. Meanwhile the Nile Delta is home to half of Egypt’s population and the country’s most important farmland.
The Mekong Delta — which produces half of Vietnam’s rice crop — is classified as being “in peril” because the delta land is sinking and sea level is rising. This is occurring even though upstream dams have reduced sediment supply by only 12 percent. The sinking and eventual loss of the Mekong Delta is yet one more reason for grave concern over the impacts of a series of proposed major dams on the Mekong mainstream as well as dam construction throughout the basin.
What can be done to reverse the sinking of the world’s deltas? Obviously, efforts to slow or halt climate change (and associated sea-level rise) will help deltas persist. Those that fund, build and license new dams should strive to minimize reductions of sediment inputs to important deltas.
Within deltas, river channels can be freed to spread across more of the land to deposit sediment, thus counteracting erosion and sea-level rise. This approach has been proposed for the Mississippi River Delta (see photo below), identifed by Syvitski as being “in peril.”
I originally wrote the words above last April, just before the Deepwater Horizon oil spill cast its dark shadow over the Mississippi River Delta and the entire Gulf of Mexico. So I shelved the post, thinking that the issue of sinking deltas would, for awhile, be subordinate to news about oil spills, impacts and recovery.
But as desperate, stop-gap actions to fight the spill have given way to long-term planning for the Gulf’s recovery, a chorus of voices is prodding the nation to use this disaster as a fulcrum for fundamental restoration of the Delta and Gulf.
Bob Marshall of the New Orleans Times-Picayune writes:
We need to remember (the oil spill ) is a temporary problem on top of a permanent disaster. Long after BP’s oil is gone, we’ll still be fighting for survival against a much more serious enemy — our sinking, crumbling delta. Our coast is like a cancer patient who has come down with pneumonia. That’s serious, but curable. After the fever breaks, he’ll still have cancer.
And Mike can point to reasons for optimism: “While Barataria Bay in the Mississippi River Delta represents possibly the greatest rate of loss of coastal marshes worldwide, just ‘next door’ in Atchafalaya Bay, the marshes are growing — and doing more than just keeping pace with sea level rise. There are important lessons to be learned here; with better river management we could re-build those marshes.” (see photo below for locations).
As politicians often say, a crisis is a terrible thing to waste.
AN AMERICAN DELTA: the delta of the Mississippi River and the Gulf of Mexico. Note that the sediment plume of the leveed Mississippi River is far beyond Barataria Bay, which is rapidly losing marsh land, while the sediment plume of the Atchalafaya River is building new marsh in Atchalafaya Bay. (Photo: modified from a NASA image)
—Text by Jeff Opperman, Cool Green Science Blog