After Hurricane Matthew churned across North Carolina earlier this month, swollen rivers deluged poultry and swine farms, killing millions of chickens and thousands of hogs and sending potentially toxic animal waste coursing into waterways.

It could take weeks or months for North Carolinians to learn the scope of the pollution or where it came from—if they ever do.

"We may never know how bad this was or the extent of the damage," said Travis Graves, a North Carolina-based member of the Waterkeepers Alliance, who has been surveying the scene from the air. "The state doesn't even know how big the poultry industry is here."

Thousands of industrial farms across the country release contaminants into the nation's water and airways, but in many states like North Carolina, the public has limited access to information about them. Federal authorities can't gauge the scope of the pollution, either, because in some states they have very little idea of the number and location of farms. This makes regulatory oversight weak and in some cases, nonexistent.

"You can get this information on coal plants or any other polluter," said Tarah Heinzen, an attorney with Food & Water Watch, an environmental advocacy group that has called for the agency to submit information about CAFOs to the public. "But you can't for this industry."