Catching a 5-kg grass carp in the Songhua River no longer comes as a surprise to fishermen in this capital city of Heilongjiang province.

One of them, surnamed Li, usually catches about 30 kg of fish every day.

It was unimaginable five years ago. In late 2005, the Songhua River, a tributary of the Heilongjiang River, called the Amur River in Russia, was severely polluted. An explosion upstream in Jilin province released a 100-ton spill of toxic chemicals, endangering the supply of drinking water for millions of residents along the river.

Following the spill, 34 percent of the river's water quality was rated Grade V or worse - the highest level of poisoning, which meant it was unfit for human use, according to environmental protection bureau records.

"Many of us thought it would be impossible to fish in the river again," Li said.

Harbin has spent 3 billion yuan ($452 million) cleaning up the river over the past five years, according to Zhou Linbo, deputy director of the city's environmental protection bureau.

A total of 19 sewage plants were built, which process 1.31 million tons of waste a day, Zhou said.

The efforts have paid off. Zhang Fengjun, assistant manager of the Harbin Ferry Company, said river cruises have become increasingly popular since the government cleaned up the river.

"The river is better now, much cleaner than it was in 2005. Residents and visitors also have greater appreciation of the natural environment and very few of them throw anything into the river during a cruise," he said.

The two provinces and an autonomous region that lie along the Songhua River - Heilongjiang, Jilin and Inner Mongolia - quickly implemented the central government's pollution prevention plan for the Songhua River in 2006.

China and Russia adopted a mechanism the same year for regularly exchanging information about the river. They also jointly launched projects to monitor and protect the river's water quality and the surrounding environment.

The two countries also set up a system for sounding the alert when pollution or natural disasters occur along the river. From 2006 to 2010, a total of 13.4 billion yuan was invested on 222 projects to combat pollution in the river.

The major objectives were to protect the supply of drinking water along the river, controlling industrial sources of pollution, the construction of sewage treatment facilities, and setting up systems for monitoring the river and alerting others in the event of an emergency.

But Zhang Lijun, vice-minister of environmental protection, warned local governments to remain vigilant and give greater attention to ecological safety in their pursuit of economic growth.