Malaysian Nature Society vice-president Vincent Chow said this was based on two studies he conducted in 2014 and 2016, which showed that some stretches of the river had become shallow and its waters black and bereft of fish.
"Local fishermen say the river used to have a lot of fish, but now it has turned black and smelly.
"I believe that illegal waste dumping has been going on for years, making this river a ticking time bomb that led to this incident, caused by chemical reactions due to the recent heat and low tide," he said.
Mr Chow said this is a wake-up call for the state government to come up with a framework to better protect the environment, not just using the resources of its own agencies, but also working in tandem with non-governmental organisations and stakeholders such as fishing communities.
He said Sungai Kim Kim was not the only polluted river in Pasir Gudang, as Sungai Tengkorak and Sungai Jelutong, both in Permas, had also become dumping grounds for waste.
"Now is the time for the government to check whether these chemicals were all from domestic sources or brought in from overseas," he said.
He added that importing waste and later dumping them into the state's rivers was a "lucrative business" for some quarters.
Mr Chow also said the Department of Environment should publish the findings of its water sampling in the area so that the public would be better informed about the current condition of the river.
Asked how long it would take to clear the river, he said it was heavily polluted, as the chemicals had seeped into the mud.
"You will have to remove all the mud and safely transport it to a safe place away from the population," he said, adding that it would take a long time.