Thai environmentalists were worried after the Mekong river changed from its distinctive muddy brown to a light ocean blue on December 4.

The Mekong river has been ravaged by droughts and water shortages, with its average height at its lowest in the last 50 years with, some parts only 3ft deep.

Bizarrely, the stretch of the river in Nakhon Phanom, northeastern Thailand, has now changed from its muddy brown shade to clear blue.

Locals and tourists crowded around the river bank to witness the phenomenon last Friday (November 29) afternoon.

The president of the Nakhon Phanom Environment Preservation Club, Arthit Panasoon, blamed China for building a dam along the river which has slowed the flow of water and deprived the river of sediment, which caused the brown colour.

He said he was worried about the ecology of the river being destroyed after this phenomenon as creatures relied on the sediment to survive.

He said: "The dam that was built by China across the border stored too much water, so that there was less flowing downstream. This caused a drought that has been the worst in the last 50 years.

"Moreover, the water that does flow, actually comes too slowly and the sediment becomes stuck on the river bed.

"This phenomenon is called the 'Hungry Water' effect and it happens when the water flows downstream without sediment, which is why it is clear. But the bad thing is that the ecosystem of this river is only familiar with living in muddy water.

"Much of the life in the river could have been wiped out by these recent conditions.''