In the heart of Weining Yi, Hui and Miao autonomous county in Guizhou province lies Caohai, the largest natural freshwater lake in Guizhou province.

At an elevation of more than 2 kilometers, the 25-square-kilometer lake has been dubbed "a pearl in the plateau".

The "pearl", home to more than 200 kinds of birds, including the endangered black-necked crane, hasn't always shone. It once shrank to only 5 sq km due to reclamation. After gaining its current size, it was threatened by unrestrained tourism and untreated domestic sewage.

Things have improved since 2015, when the county started a campaign to repair Caohai's environment.

According to local officials, the county has paid local farmers for 4,000 hectares of water, wetland and farmland to carry out protection projects, including wetland restoration and building foraging habitats for birds.

Further, the county banned tourism at the lake in 2019.

Xia Chaowen, an official of the management committee of the Caohai national nature reserve which covers 120 sq km, said visitors used to take the farmers' boats to tour the lake, resulting in litter, alarmed birds and damaged aquatics. Chemical fertilizers and pesticides used by the farmers posed another hazard to the water, Xia said.

The county has also moved more than 2,300 households from the lakeside.

Located next to the county's traditional communities, Caohai can be easily affected by human activities. Before the overhaul campaign started in 2015, residents living around the lake used to discharge sewage directly into the water.

As a result, the county built a sewage treatment plant on the upper stream of Caohai, processing 8,000 metric tons of sewage every day on average, as well as 19 smaller ones around the lake.

During an interview in June, Hong Huining, deputy head of the major treatment plant, showed the media how sewage is treated in her factory.

The wastewater becomes clear and odorless after a number of steps to remove impurities. In the end, the water runs through an artificial wetland filled with different plants used to further absorb harmful substances before being discharged into Caohai.

Positive results have surfaced after five years of restoration efforts.

Li Zhenji, an official of the environmental monitoring department under the Caohai reserve management committee, said 246 kinds of birds were recorded in the region this year, 18 more than in 2010.

The overhaul campaign has improved biological diversity in Caohai, leading to a more stable and stronger biological system, according to Li.

Plants such as water fern, which used to "grow wildly" over Caohai due to deteriorated water quality and disrupted growth of aquatic life, can hardly be seen in the lake now, while some other plants rarely seen in the past have become more common in recent years, according to Li.

Located next to Yunnan province and near Sichuan province, Caohai is part of the Yangtze River system. It is located at the upstream of the Hengjiang River, which is a branch of the Jinsha River.

As a result, protecting Caohai is not only important for Guizhou, but also for the environments of neighboring Yunnan and Sichuan, according to Li.

Hong, who works in the sewage treatment factory, recalled that the water in Caohai worsened in the 1990s, and she was "very disappointed" to see the lake turn "black and smelly".

With the water quality improved significantly since 2015, Hong said now she and her husband often take walks by the lake to enjoy its "pleasant view", especially the "good air and clear water" in the summer.

Hong said she is confident that Caohai will continue to get better and added that the lake should be protected by all local people.

Yang Jun contributed to this story.