Workers deal with water lettuce on a refuse-removal boat in Chongqing on Sunday. [Photo by Ran Wen/For China Daily]

After heavy rains on the upper reaches of the river last week, the plants washed into Chongqing on Saturday at about noon. Large amounts soon gathered in the middle of the river and along the banks.

On Tuesday, the city's maritime department issued a warning and asked boats to avoid the floating plants.

The species, introduced from overseas, is among the world's most productive freshwater plants. It is found in nearly all tropical and subtropical freshwater areas.

As it often grows into dense mats that clog channels, China has listed it as one of its 100 most dangerous invasive species. In addition, water lettuce will also block the exchange of air, reducing oxygen in the water and killing fish. Large mats can also block light and alter underwater plant communities.

At present, the main way to control water lettuce is to physically remove it and transport it to onshore disposal areas.

This is "the second time this year" that water lettuce has been found in downtown Chongqing, said Di Hongwei, of Chongqing Water Environment Co, which cleans the city's waterways.

The company scoops out garbage and storm debris to make waterways more scenic and easier to navigate.

Flood season is the busiest time of year for river cleaners. Last month, the company removed more than 1,000 metric tons of water lettuce from the river. The work took a week.

"We didn't find water lettuce last year," Di said. "But there was a large quantity of it in 2015. In some backwater bays, the plants were so dense that a man could stand on them."

To speed up the cleaning, the company has sent three large refuse removal boats and hired another four to remove the plants around the clock. It expects to take a week to finish the work.

Chongqing, a megacity of 30 million people, is located on the upper reaches of the Yangtze, the world's third-longest river at 6,300 kilometers.

Since the Three Gorges Dam was built, the river's water level rises in winter and drops in summer, opposite the natural cycle.

Garbage on the river's surface creates unpleasant scenery and difficult navigation for ships. Trash can even affect the production of electricity at the dam. In recent years, Chongqing has removed more than 5 million tons of garbage from the river.