Layers of trash floating in the Yangtze river are threatening to jam China's massive Three Gorges hydro-electric dam.

Chinese state media reports the garbage is so thick in parts of the river that people can walk on the surface.

China Daily says nearly three tonnes of refuse are collected from the dam every day, but operators are struggling with inadequate manpower and equipment as rubbish accumulates more quickly due to rain-triggered floods.

"The large amount of waste in the dam area could jam the mitre gate of the Three Gorges Dam," China Three Gorges corporation official Chen Lei said in the newspaper.

More than 150 million people live upstream from the dam.

In several nearby cities, household garbage is dumped directly into the river - China's longest - because municipalities are not equipped for trash disposal.

Mr Lei said 160,000 cubic metres of trash was collected from the dam last year.

The newspaper says the China Three Gorges corporation spends about 10 million yuan ($1.6 million) per year to clear floating waste.

According to state newspaper Hubei Daily, a 60-centimetre thick layer of garbage covering an area of more than 50,000 square metres began to form in front of the dam when the rainy season started in early July.

China considers the $22 billion Three Gorges Dam a modern wonder.

Since its completion in 2008, it has pumped out much-needed hydro-electricity, increased shipping on the Yangtze and helped reduce flooding.

But critics say the world's largest dam has caused ecological damage and increased landslides in the area.

About 1.4 million people were displaced when the dam was built and several heritage sites have been put deep underwater.