Most of the world's largest rivers are losing their connection to the sea, with nearly a quarter of those left risk being disconnected in the next 15 years, the WWF has said.

According to a new WWF report, only a third of the world's 177 large rivers (1,000km and longer) remain free-flowing, unimpeded by dams or other barriers.

Only 21 of these actually run freely from source to sea, the other 43 are large tributaries of rivers such as the Congo, Amazon and Lena.

The report, Free-flowing rivers - Economic luxury or ecological necessity? - shows that the ever increasing loss of free-flowing rivers is a disturbing trend, threatening the supply of water for drinking, sanitation, agriculture, fish and fishery products.

"With so few long free- flowing rivers left, we are on the brink of losing another natural phenomenon without fully understanding the costs of these losses before it’s too late," said Ute Collier, WWF's Dams & Water Infrastructure Programme Manager and co-author of the report.

According to WWF, the threat to wildlife through damming rivers cannot be understated. Large catfish populations in the Amazon and Mekong Basins, river dolphins in the Ganges Basin and wildebeest in the Mara River are all under threat from the effects of man-made barriers on these rivers.

Dams can reduce the numbers of native fish in a river, directly affecting fisheries productivity both upstream and downstream. Free-flowing rivers also regulate pollution and sediment levels, the lack of which was tragically highlighted by the flooding of New Orleans following Hurricane Katrina.

"Hurricane Katrina was a powerful reminder of the backlash from altered rivers such as the Mississippi," said Jamie Pittock, Director of WWF's Global Freshwater Programme.

"Loss of the sediment needed to sustain coastal wetlands due to upstream damming and canalization of the river is a major factor in the devastation and loss of life."

Most large free-flowing rivers are found in Asia, followed by South and North America. Elsewhere, the Australia-Pacific region has the fewest, with just three, and in Europe, including areas west of the Ural River, only one large river, the Pechora in Russia, remains free-flowing from source to sea.