Sea-level rise and river engineering 'spell disaster' for many of the world's river deltas, say scientists.
Half a billion people live in deltas, but the newly published research suggests many of these areas are set to be inundated by rising seas.
Some of the lowest lying, including the Mekong and Mississippi, are particularly vulnerable.
The paper is published in the journal Nature.
Lead researcher Dr Liviu Giosan, from Woods Hole Oceanographic Institution in Massachusetts, said dams and other river engineering had exacerbated the problem by reducing the amount of sediment rivers could carry.
In an article he said was 'a call to action before it's too late', Dr Giosan said rivers were losing the fight between land and sea.
'Rivers produce sediment by eroding the land, and the sea [washes away] that sediment,' he told BBC News.
A delta is the result of a river producing sediment faster than the sea can remove it, resulting in expanses of fertile, flat land. These are home to some of our most sprawling megacities, including Shanghai, Dhaka and Bangkok.
And where they are left pristine, they are also sites of great biodiversity. One of the world's better preserved deltas, for example - the Danube - is the most extensive wetland in the European Union and a global biodiversity hotspot.
Although the UK has estuarine areas - where rivers flow into brackish coastal areas that have an open connection to the sea - these do not produce enough sediment or flow out on to large enough flats to build a delta.
Battling the sea
Investment now will let nature - the river, beaches, marshes and mangroves - adapt on their own to the sea level rise and keep deltas alive at little cost'
End Quote Liviu Giosan Woods Hole Oceanographic Institution
Dr Giosan and his colleagues used historical measurements of sediment deposits from delta-building rivers and compared them with the sediment needed to keep up with projected sea level rise in the 21st Century.
This suggested that most large and medium-sized deltas would lose the fight with the sea - unable to 'trap sufficient sediment' to remain above sea level.
'All deltas will be affected and their area will be reduced,' said Dr Giosan.
'But lower lying deltas such as the Mekong, Mississippi or Danube will be more extensively inundated than others if maintenance measures are not stepped up.'
These measures, he said, included building better dams and modifying old ones 'to let more sediment reach deltas'.
'We can trap more sediment in the delta rather than let it waste in the ocean, [and] we can support marshes to keep up with sea-level rise,' Dr Giosan told BBC News.
'Investment now in these activities will let nature - the river, beaches, marshes and mangroves - adapt on their own to the sea-level rise and keep deltas alive at little cost.'
Dr Giosan and his colleagues are calling for the United Nations to establish an international body of experts to coordinate delta-maintenance initiatives worldwide.
The costs of doing nothing, he said, would be huge, with vast areas of land underwater, harbours unable to allow shipping and rich agricultural land destroyed by salt.
'We [would] lose biodiversity, ecological services, damage economies and trade, and force people to migrate by not planning ahead.'
Prof Paul Whitehead, an expert in river dynamics, from the University of Oxford, said the article was 'fascinating' and summarised some key findings in deltas around the world.
'My experience of working in the Ganges-Brahmaputra-Meghna Delta in Bangladesh is that the same processes are at work,' he told the BBC.
'Over half the world's population live in flood plains or in deltas - that is why it is so important.'
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