A lake in southern Beijing has been transformed from a black, malodorous water body into a vibrant habitat for thousands of birds, thanks to local authorities' efforts in promoting a "sponge city" program.

Tongming Lake in the town of Yizhuang, an economic and technological development area in Beijing, used to be "befouled by bad smells", said Huo Ke, who lives in a nearby community.

Nowadays, however, visitors to the 43-hectare lake will find a large flock of birds, sometimes reaching 10,000 in winter. The number is so large that, when some of them take off together, they form a moving "cloud", said Huo, a keen bird watcher who regularly visits the lake.

"It's so small an area with such high a density of birds. I think this is really rare in Beijing," he said.

The positive changes happened after Yizhuang authorities initiated the sponge city program in 2013 designed to soak up as much extra water as possible with the help of storage tunnels, permeable pavement, rain gardens, ponds and wetlands.

Developed into a flood storage area, the lake was designed to help accommodate rain water from a nearby area of 7.6 square kilometers.

For the following almost five years, the lake was enclosed to protect the birds' habitat and prevent people from swimming and fishing, according to local authorities.

With more measures taken this year, the lake was built into a public park and has been open to the public since late September.

Aside from removing 50,000 cubic meters of silt from the bottom of the lake, the local government also used about 120,000 cubic meters of crushed construction waste to pave the lake's shore area, making the area more permeable. Many aquatic plants, mostly reeds, have also been planted near the shore, said Qian Peng, an official with the transformation project.

In addition to rain water, which flows into the lake via a tunnel, the local government also discharges reclaimed water into the lake, he added.

Qian said the lake now boasts almost 200 species of birds, though some of them, including many types of wild ducks, only visit during winter.

Yizhuang authorities say Tongming Lake is only one of seven public spaces especially built or transformed in the sponge city program. The others include roads and squares.

Projects have also been rolled out in 240 blocks of industrial, commercial and residential areas.

In total, 1,200 hectares in the economic and technological development area are now covered by the program.