Much has been made of the benefits of bike-sharing: Reducing congestion, vehicle emissions, and fuel consumption -- which in turn lead to health benefits and financial savings.

But it also has a dirty secret: The environmental impact of the increasing number of bikes that are being abandoned.

Two bike-sharing companies in southern China have recovered more than 3,000 bicycles from rivers during clean-up operations. (Picture: ThePaper.cn)

The South China Morning Post reports that two Chinese bike-sharing firms have pulled more than 3,000 bikes out of rivers during clean-up operations in southern China.

Mobike found more than 1,000, while competitor Ofo recovered nearly 2,000.

(Abacus is a unit of the South China Morning Post, whose parent company Alibaba backs Ofo.)

Mobike told local media that it appears to be mostly the work of vandals.

But this is only the latest development in the ongoing saga of bike-sharing -- caused in large part by the over-supply of millions of new bicycles that have hit Chinese roads over the past few years. At one point there were as many as 16 million shared bicycles on the road in China.

At the height of the bubble, there were close to 60 dockless bike-sharing start-ups, whose colorful bikes were seen being dumped outside subway stations, causing daily headaches for local governments.

People riding Ofo’s yellow bicycles try to pedal along a pavement crowded with bicycles from various bike-sharing companies in Beijing last month. (Picture: South China Morning Post)

Then there’s the environmental impact. Among the victims of China’s bike-sharing bubble was Bluegogo, which was once the country’s third-largest bike-sharing start-up -- with 600,000 bikes across China.

Bicycles of various bike-sharing companies are seen in Hangzhou, Zhejiang province. (Picture: South China Morning Post)

So where do all these bikes end up?

Those that aren’t sitting at the bottom of rivers find their way to “bicycle graveyards” -- huge fields that house piles of bikes that authorities have collected.

One such park in Nanjing had tens of thousands of bikes piled up to a height of nearly two meters.

The problem of abandoned bikes also exists overseas, albeit on a much smaller scale.

In Australia, shared bikes have been spotted up trees, on lampposts, and even on top of bus stops.

Ofo and Mobike told Chinese media they plan to revamp their apps to encourage cyclists to park responsibly. Ofo, for example, deducts credit points from customers for poor parking. Mobike also said it will try to recover usable parts from the salvaged bikes.