More than 500 million people live in the world's river deltas, yet new research shows that these population magnets are under increased pressure from human activity.
Fish trader Lê Thanh Liêm thinks he might have to take down his wooden house again and move it to a safer place. The middle-aged Viet Cong veteran has already moved three times since he went to live in Cà Mau, the southernmost region in Vietnam, in 1978. But the annual floods keep getting more severe as the Mekong River Delta steadily erodes. The last time, villagers were up all night carrying things to higher ground, he says. As soon as the water receded, they rushed to clean out the mud before it dried.
“If the water keeps getting higher each year, we will move further inland and build the house even higher,” says Liêm.
He might well be forced to do that. Deltas – some of the most densely populated areas in the world – are sinking and becoming harder to live on. In Cà Mau, groundwater exploitation and deforestation have eroded the coastline by between 100 and 1400 metres over the past 13 years, according to a recent satellite-based study by Norway’s Geotechnical Institute (NGI) and Vietnam’s Ministry of Agriculture and Rural Development. The land surface has fallen by up to 70 centimetres in places, they found.
“The subsidence may reach 80 to 150 cm within the next two decades, but will also continue to increase after that,” said the NGI in a statement in June 2013. “Since the average surface of the Cà Mau province is only 1 to 1.5 metres above sea level,” said NGI Technical Director Philos Kjell Karlsrud, “the consequences of such subsidence will be catastrophic, and make the province inhabitable within a few decades.”
Many of the world’s deltas are subsiding by 10 cm or so a year – far faster than the 3 mm or so annual rise in sea levels – causing the sea to swallow up dozens of metres of land each year. In the past decade, 85% of the world’s major river deltas experienced severe flooding, killing hundreds of thousands of people.
The biggest, and best known, cause is upstream damming. This prevents new sediment running down from inland and settling on top of soil that naturally compacts. That's compounded by extraction of liquids from underground – mostly water but in some cases oil or gas.
More recently, research is beginning to highlight the impact of farming and other human activities on delta coasts. Cutting down mangroves – often to make shrimp farms – removes a natural defence against waves and storm surges that can pull loose soil into the sea. Aquaculture often uses underground water too. Over the past decade, satellite-based monitoring techniques are revealing this damage in new detail: a July 2013 paper in Geophysical Research Letters presented possibly the first concrete evidence that aquaculture had caused subsidence on a coastline by pumping water from compressible sediments underground.
“There has been a rapid improvement in technology over the last 15 years,” says James Syvitski, a professor at at the University of Colorado at Boulder, who is also chair of the International Geosphere-Biosphere Programme. “In some cases we thought subsidence was due to gas removal, but we discovered it was fish farms instead. The trouble is, they won't have fish farms for the next generation.”
Deltas are formed when great rivers wash sediment from inland down to the coast, creating a coastal bulge in the shape of the Greek letter. The terrain is typically flat, with only a few metres difference in elevation over 100 km, and the sediments are rich in nutrients. That makes the land rich in biodiversity and ideal for farming, helping turn deltas into major sources of rice and other crops. In addition, they are often crisscrossed by a network of waterways with easy access to oceans, making them natural commercial hubs.
As a result, deltas such as that of the Nile were the locations for some of the first great civilisations. But the real population explosion has come in the last half century. More than 500 million people now live on deltas – all within 5 m of sea level – even though they occupy just 1% of the Earth’s land area.
Deltas are naturally dynamic geological structures. Though sediment from upstream settles and is gradually compacted, a steady arrival of new sediment maintains the land level and expands the delta. Human activity actually boosted the growth of deltas in the past, as farming, mining and logging caused soil to erode, after which it was carried downriver. In consequence, some former sea ports are now several kilometres from the coast.
But the water control systems that accompanied the industrial revolution – such as levees, canals and sluice gates – slowed down the flow of sediment. More recently, dams have all but halted the arrival of new material to the deltas of the Nile, Indus and Yellow rivers.
Asian river deltas have also been damaged by a series of effects related to mangroves and fisheries. Booming demand for seafood, in particular shrimp, has led to an explosion in Asian aquaculture, which accounts for 89% of world production. Asia’s total aquaculture production in 2010 was 53 million tonnes compared to 11 million tonnes in 1990, according the United Nations Food and Agriculture Organization.
The aforementioned Geophysical Research Letters paper, by Stephanie Higgins of the University of Colorado in Boulder and others, presented measurements using InSAR – a satellite-based radar technique – that showed that groundwater extraction at aquaculture facilities was causing China’s Yellow River delta to subside. Though this had long been suspected, it had not been shown conclusively because of the difficulty of taking accurate readings of ground levels along a muddy, flooded coast. The measurements showed that “shrimp farms can contribute to subsidence directly, if groundwater is pumped from compressible sediments to sweeten the ponds,” said Higgins in an interview.
Mangroves, in addition to acting as a barrier against the sea, trap sediment and bulk up the land with root volume. In Vietnam, they have been depleted heavily. At first that was due to the demand for wood. Cà Mau was thick with mangroves when Lê Thanh Liêm, the fish trader, arrived 35 years ago after leaving the army and marrying a woman from the area. But people cut down a lot of the trees for charcoal, chopsticks and building. “Poor people went to the forest to burn charcoal,” he says. “They could earn about 25 US dollars a day. They understood that cutting down trees will cause land to be lost, but they were hungry and needed money. They had no choice”.
Later, in the past decade or so, farmers have been cutting down mangroves to create space for their shrimp ponds. Various surveys indicate that roughly half the mangrove forest on the Mekong Delta has disappeared over the past three to five decades, and the current area is 100,000 ha or less.
Liêm used to be able to hear a strong south wind before he could feel it. “Back then, you knew there was a wind but it took 10 or 15 minutes until you could feel the breeze because the trees slowed the wind down,” he says. “Now, you can feel the wind immediately.”
“If you ban shrimp farms a lot of small people will lose their livelihoods,” says Steve Trent, Director of the Environmental Justice Foundation (EFJ). “But as a net economic contribution over the longer term, they are basically undermining the future basis of the business.”
Mangrove planting programmes have been launched in recent years, including one funded by Germany. In 2006, the government restricted to 40% the amount of mangrove that can be cut down to make shrimp farms or canals. “In the past farmers cut down mangroves for shrimp as much as they wanted,” says Nguyen Van Hao, Director of the Research Institute for Aquaculture No 2 (RIA2) in Ho Chi Minh City. “Now they must live together with the mangroves and cannot cut them all down.”
That’s not popular in areas where shrimp have become the road to prosperity. In Dat Mui, a town on the southern extremity of Cà Mau, the air smells of shrimp left out to dry. People talk about shrimp. And some have managed to buy land with money made from shrimp.
Nguyễn Văn Đá, who recently abandoned fishing after a stroke, has not yet managed this in spite digging a shrimp pond in front of his wooden house 15 years ago. The river that runs nearby, the Rach Mui, was so narrow 10 years ago that even a small boat could not turn round, he says. But now it is five times wider, as the banks have collapsed into the water little by little. He blames the speed canoes that have started bringing tourists to the area: “They create strong waves that tear off the land,” he says.
The government, in contrast, thinks the problem was farmers cutting down mangroves. Dá was fined 2 million VND, about $100, for this. “Even when the trees grow into the shrimp farms you cannot cut them away,” he complains. “They made too many regulations. My dream is a shrimp farm to secure my life. But there is no way to expand or make another shrimp farm without violating the law.”
In other countries, measures to save deltas include smarter dams that trap less sediment and restrictions on the pumping of underground liquids. This was done at the mouth of Italy’s Po River, south of Venice, where methane extraction caused subsidence, and around Bangkok, where the shoreline of the Chao Phraya River delta retreated due to water pumping.
Still, action is complicated by competing interests. In 1995, Cambodia, Laos, Thailand and Vietnam created the Mekong River Commission to promote shared use of the river. Though the Commission is widely considered to have improved coordination, it does not have a mandate to resolve disputes over damming, for instance. The Lao government is currently building a dam in Xayaburi in northern Laos, to generate electricity. Cambodia and Vietnam – further downstream – oppose the project, because they fear its effects on their fisheries and cropland.
In the meantime, life is getting hard for the people who live on deltas. Sometimes that’s because of deadly storm surges, such as the one that killed around 200,000 people in the Irrawaddy Delta in 2008. Sometimes it just gets harder to make a living.
Huynh Van Gio, 65, has been living in Hon Dat, a town in Kien Giang – a province to the north of Cà Mau – since 2006. He came from his home in An Giang province, further inland, as he’d heard there were plenty of fish to catch. Each year since then, more land has been eaten away by the sea water. He says the erosion happens when strong, southerly winds blow from April to June, creating high waves that batter the land.
Between his house and the sea, a mud road that used to lead to the sea stops abruptly where a new inlet has formed. Now people have to take a detour if they want to get to the sea, he says. He points to a shrimp farm about 10 metres from the sea. “The owner of this shrimp farm is very worried,” says Gio. “He bought and planted small trees to protect the land, but I don’t think this will work. Even big trees have been dragged into the water.”
Gio has had enough of life on the delta. “When I came, the sea was very far away,” he says. “Since then the sea has come about 100 metres closer and swallowed trees that had been planted to form protection from the water. Now it's only about 100 metres away from our home. Just a few weeks ago, about three of four metres of land fell into the sea. The question is when it will come into our house”. He's decided to leave next year and move back inland.