Is this ‘age of the delta’ coming to an end?

The land near the mouth of the Mississippi River is barely land at all. Muddy water forks into a labyrinth of pathways through a seemingly endless expanse of electric-green marsh grass, below skies thick with birds. Shrimp and crabs wriggle in the water below, and oak and cypress sprout from wet soils on higher grounds. Stretching for more than a hundred miles along the coast of Louisiana, this is one of the world’s largest, and most famous, river deltas.

Similar landforms — flat and wet and rich with wildlife — have built up wherever rivers dump their mud and sand into shallow oceans. There are thousands of such deltas across the world, ranging in size from just a few acres to, in the case of the Mississippi River’s, thousands of square miles. Many of these places, including the marshes in Louisiana, are now disappearing, often quite rapidly.

The soils are sinking, for varied and complex reasons. Worse, as the climate warms, the seas are rising. In Louisiana, a football field’s worth of land is gone every hundred minutes, a fact that is devastating for the hundred-million birds that stop here on their migrations — and for the people whose families have lived amid these wetlands for generations, and sometimes millennia. Hundreds of millions of people that live on other deltas, too, could lose their homes and their livelihoods.

Later this year, a state agency in Louisiana will begin construction on what has been called the largest ecosystem restoration project in US history. A set of gates in the levee will allow water to once more carry mud into the marshes, an effort to rebuild tens of thousands of acres of land. The project has been controversial, particularly because it’s likely to affect fisheries, a major local industry.

Deltas are inherently impermanent: As climate changes through the epochs, as the seas rise and fall, deltas are formed and then buried underwater. Still, these past few thousand years can be thought of as an “age of the delta,” says Utrecht University geomorphologist Jaap Nienhuis. Not coincidentally, it has also been an age of human flourishing. “Human civilization started when modern river deltas started to appear,” Nienhuis says. “It’s scary to think of what could happen if river deltas disappear.”

Nienhuis, lead author of a 2023 paper on river deltas for the Annual Review of Earth and Planetary Sciences, spoke with Knowable Magazine about what deltas face in the coming centuries, and whether strategies like Louisiana’s “sediment diversion” can make the needed difference. This interview has been edited for length and clarity.

What are river deltas, and why are they important?

River deltas are places where river sediments settle. Rivers slow down when they reach the sea, and then all the sediments that rivers carry are dropped out. That builds land over time.

A typical delta is a place that’s flooded, sometimes by tides every couple of hours, while some deltas flood only once a year, during a monsoon or a big hurricane. The ecology and biology have to be very well adapted to this periodic inundation.

So what we typically find are swamps: cypress wetlands or mangrove forests or salt marshes, which are well-adjusted to dealing with inundation by tides. These are great places for fish. The river-carried sediment is rich with organic material, which is good for agriculture. So it’s not a surprise that many population centers are on river deltas: New Orleans, Shanghai, Tokyo, Jakarta. Much of the Netherlands is a delta. In total, 500 million people live atop river deltas across the world.

How do deltas form?

Deltas exist on a balance. Some forces are constructive. Basically, you need the river to supply a lot of sediment. That can happen naturally, through mountain-building and then erosion — that’s why we have a lot of big deltas downstream of the Himalayas.

Then there are destructive forces. Sea level is a first-order control on the evolution of deltas. When the seas are rising, it’s hard for rivers to supply enough sediment to keep up.

To understand today’s deltas, we have to go back 20,000 years, to a time when most of the planet was covered by ice sheets. A change in the Earth’s orientation toward the sun kickstarted a melting of the ice and a rise in sea level. So from 20,000 years ago to 10,000 or 8,000 years ago, we had a long period of very quickly rising seas. There wasn’t a lot of delta-building happening, and older deltas were being smothered by the water.

Then, when most of these ice sheets were nearly melted, the rates of sea-level rise began to slow. That’s when all the sediments that came out of rivers were able to build the modern deltas. That happened for most river deltas roughly around the same time — 7,000 years ago. Of course, when they first appeared, these deltas were very small. They’ve built over time.

Interestingly, the start of several ancient civilizations and ancient cities coincides with that deceleration in sea-level rise. This is not a coincidence. Humans and deltas are very intertwined.

It sounds like our species has depended on deltas throughout our recent history. Have we also affected deltas?

We have. Humans can actually be a constructive force for deltas. What we’ve seen over the past few hundred years is that deforestation upstream in a river’s watershed can cause an increase in the amount of sediment that is brought down by a river. So in that sense, humans have greatly accelerated delta growth over the past centuries.

But humans can be a destructive force, too. There is subsidence, for example: land lowering, which happens a lot in deltas because they are built from young, soft soils.

Here in the Netherlands, we started to drain the delta because we wanted to use it for agriculture. And then subsidence accelerates and the land sinks beneath your feet, since the water in the soil dries out. So now you build levees to protect yourself from all the extra flooding.

But now, because of the levees, sediments are no longer supplied to your part of the delta, so you sink even further. So you build more levees. The reason why the Netherlands — and also parts of New Orleans — are below sea level is because of that cycle. You create a situation you cannot get out of. We call that a “lock-in.”

If the constructive and destructive forces are in balance, deltas will stay the same. But it’s a very, very, very thin balance. And humans, most of the time, we’re destructive.

Do we have a sense of how big of a problem subsidence is?

We have a very good overview of all the things that could cause subsidence. But it’s very hard to measure it.

Subsidence happens on many scales at once. If ice sheets melt in, to give just one example, Canada, that can take weight off the Earth’s mantle, causing it to tilt. That means that as the ice melts — and land lifts — in Canada, the land as far away as the Mississippi delta can wind up sinking. So there are changes happening on a scale of thousands of miles.

At the same time, if you build a house and add weight to the soil, you also sink the soil. Even just standing on it, jumping on the ground — you’re changing the pore structure in the grains of soil below.

Imagine trying to also integrate what vegetation does with its roots, plus worms that go through the ground. Then you have oil extraction or taking out groundwater — changes on a medium scale. So it’s hard to validate and test our models. It’s very much a limiting factor in our ability to say something about the future of deltas.

Really, though, given how big a control sea-level rise is on delta construction, the biggest problem now is climate change. As ice sheets begin to melt again, that’s causing seas to rise much more quickly than they have in thousands of years.

It sounds like we’ve been in an “age of the delta,” but that’s now coming to an end.

Definitely. If we zoom out and look at deltas as a whole, we could lose as much as 5 percent of the land by the end of this century, mostly because of sea-level rise. And as much as 50 percent of the land — 130,000 square miles — in only a few hundred years.

So it could go very, very fast. Since sea level is so important here, a lot of that depends on what we do in terms of CO₂ emissions and climate change.

We’re not going to lose every delta. When deltas are very small, and uninhabited by humans, it’s very easy for them to respond to sea level. Think of Arctic deltas, for example. Basically, when seas rise, these deltas walk with sea level up the river — the delta advances upstream, replacing what was once solid ground with wetlands.

But for many deltas, we’ve built levees, so they’re fixed — they’re not moving anywhere. And big deltas are hard to sustain. They need a lot of sediment. Those deltas are much more uncertain.

Eventually, sea-level rise will slow again, and then we’ll reenter an age of the delta, when rivers can build again. Ironically, if we continue emissions at a high rate that might happen sooner, because we will speed up the tipping point where the world’s ice sheets collapse. If we continue emissions at a high rate, then sea-level rise might slow again around the year 2400, and deltas may be able to grow again by the end of this millennium, once much of Greenland and Antarctica have melted.

Though with seas 20 meters higher, they’ll be in new places. The Mississippi delta will be built somewhere close to the border with Arkansas. The Dutch Rhine delta will be in today’s Germany.

Is there anything we can do to save today’s deltas?

There are adaptation methods that can work for individual deltas. One key idea is trying to use the sediments coming down from the river in a smarter way. That typically means breaking down levees and restoring the natural flow of the river into the delta’s wetlands. These projects are sometimes called “river diversions.” The biggest projects — both in terms of cost and in terms of how much sediment is delivered to the delta — are happening in the Mississippi River delta.

But even if you include all the projects that are still in the planning stage, these kinds of restoration strategies are being applied to save 0.1 percent of the world’s deltas. That’s very small compared to the potential loss of land.

I’m a little bit skeptical about our ability to scale these strategies up and solve the whole delta problem. River diversions lead to high sedimentation rates, but these rates decline over time. The sedimentation rate is dependent on the elevation difference, so as you build your land, it gets harder and harder to keep up. And diversions work on very small parts of a delta. If one sediment diversion works very well in the Mississippi, you can’t say, “I’m going to build another right next to it.” At some point, you’re going to run out of sediment.

Most river deltas will persist throughout my lifetime. But to think about my daughter’s lifetime, or my grandchildren’s — that’s going to be very, very different. It’s very strange to think about these places not existing.

That’s a dark future. Is there any reason to be optimistic?

If we really stick to the Paris Agreement, an international treaty that aims to keep global temperature rise below 2 degrees Celsius, then the picture is going to be much more nuanced. Then the optimism I have is that the age of the delta will last a long time still.

It’s not really a question of whether we’ll lose delta land, but more a question of when we’ll lose it. If we really limit our emissions, we might be able to postpone the major losses for 500 years, or a thousand years — and by then, who knows? We might all be living in floating cities!

The optimism has to be that it goes slow enough that we come up with new solutions. But in order to give ourselves the time we need, we have to start addressing climate change now.