BEIJING (AP) — Intense flooding has swept thick layers of garbage down the Yangtze River that are threatening to block the gates of the Three Gorges Dam, state media reported Monday.

"The large amount of waste in the dam area could jam the miter gate of the Three Gorges Dam," dam official Chen Lei told the official China Daily in an interview, referring to the dam's huge shipping locks.

The report did not elaborate on the effect, but if the dam's gates become blocked, operators likely would have difficulty opening and closing the locks to control shipping on the river.

China's worst flooding in a decade has killed at least 991 people, left 558 missing and caused tens of billions of dollars in damage, the State Flood Control and Drought Prevention office said. More heavy rains were expected through Tuesday.

Chen said heavy downpours have pushed unusually large amounts of garbage downstream, including tree branches, plastic bottles and other domestic waste. Nearly 3,000 tons (6 million pounds) of garbage are collected from the dam daily, but there is not enough manpower and equipment to clear it all, he said.

A layer of garbage about 60 centimeters deep (nearly 2 feet) covering an area of more than 50,000 square meters (about a half million square feet) began to form in front of the dam when the rainy season began in early July, the China Daily reported, citing the Hubei Daily newspaper. In some areas, the trash is so thick that people can walk on it, it said.

"Such a large amount of debris could damage the propellers and bottoms of passing boats," Chen was quoted as saying. "The decaying garbage could also harm the scenery and water quality."

The Three Gorges Corp. has spent some 10 million yuan ($1.5 million dollars) to clear about 150,000 to 200,000 cubic meters (5.2 million to 7 million cubic feet) of floating waste from the dam area annually, the paper said. More than 150 million people live near the dam and its upper stream, but many cities remain unequipped for garbage disposal.

The dam, the world's largest hydropower project, was promoted as the best way to end centuries of floods along the Yangtze River basin. Government officials long ignored complaints about the enormous environmental impact of the $23 billion reservoir that has displaced more than 1.4 million people.

Meanwhile, the death toll from a bridge collapse triggered by intense flooding in central China has risen to 51 people. Rescuers searched for 15 people still missing in Henan province's Yi River after a bridge collapsed more than a week ago, the official Xinhua News Agency reported Monday. A witness said the bridge collapsed when pedestrians crowded onto it to watch the flooding after heavy rains overflowed the river.

Over the weekend, about 300,000 residents were trapped and left without tap water in their homes in the northeastern province of Jilin after torrential rains drenched the industrial city of Tonghua and damaged four major water pipelines, Xinhua reported.

Flooding has hit areas all over China this year. About 875,000 homes have been destroyed, 9.61 million people evacuated, and 22 million acres (8.76 million hectares) of crops ruined, according to the state flood control office.