Beaches in Indian River, St. Lucie and Martin counties were closed Tuesday, and health officials warned people to avoid swimming in or eating fish from the Indian River.
Authorities estimated the spill at six to seven million gallons. It will be weeks before they can determine the full impact of the spill, scientists and health officials say.
There has been only one larger raw-sewage spill in recent history -- 20 million gallons in Palm Beach County in September 1990. There was a six-million-gallon spill in Miami in 1991.
The spill had its origin in a small leak that appeared Saturday night in a 36-inch pipe at the Fort Pierce Utilities Authority Wastewater Treatment Plant, just off the Fort Pierce Inlet.
Workers battled the leak over the weekend, but, because of Hurricane Irene, there was a shortage of big portable pumps capable of handling such a load.
The hole grew steadily.
``Around 3:30 p.m. Monday, the leak had widened to the extent that they could no longer control it with pumps,'' said Brad Russell, regional waste-water program manager for the state environmental agency.
The spewing sewage threatened to undermine the plant's main control room. Faced with the potential loss of the entire system, Russell said, workers had to shut down the pipeline and divert raw sewage into the nearby river.
Given that the problem could have become worse, Russell said, he had no criticism of that decision, nor of how the plant dealt with the crisis. The iron pipe was only 15 years old and should not have failed, he said.
The entire Fort Pierce sewage system was heavily infiltrated with water from Irene, and the Indian River -- really a slow-moving, shallow lagoon between the mainland and the Treasure Coast's barrier islands -- was already under stress because of millions of gallons of polluted fresh water runoff from Irene.
The sewage spill made matters worse, not only by dumping untreated sewage into the water but by upsetting the delicate brackish quality of the river, which is home to a diverse community of marine life unseen elsewhere in the U.S. The estuary is the spawning ground for millions of fish and crustaceans and hundreds of manatees.
``We already have a crisis situation,'' said Brian LaPointe, a research scientist with Harbor Branch Oceanographic Institute. ``With a raw sewage spill like this, a crisis could go to a catastrophe.''
``As long as there is any threat of fecal coliform [bacteria] people should avoid contact with the water or fish from that water,'' said Gary Roderick, southeast Florida regional environmental administrator for the Department of Environmental Protection.
Fecal coliform causes stomach disorders, nausea, vomiting and diarrhea.
``We have no idea how far it has spread, or how much contamination there might be . . . so we have to err on the side of caution,'' said Diane Walgren, administrator of the St. Lucie County Health Department.
Oxygen levels in the contaminated water will likely go way down, LaPointe said, because it is used up by bacteria digesting the waste. That, in turn, could cause fish kills.
A potential long-term effect of the spill: The heavy load of nutrients would release the equivalent of a super fertilizer, causing a ``harmful algae bloom'' that will smother other plant life in the river.
Locals and tourists were disappointed.
``I was looking forward to swimming in the cove and partying as usual,'' Patricia Koons, 28, said as she stood near her car at Fort Pierce Beach. ``What's the fishing going to be like? What will this do to tourism?''
Down the road, Stacey Hampton, owner of Capt. Jim's Bait and Tackle, was already feeling the pinch of bad news. ``If nobody's fishing, nobody's buying bait, nobody's making money and nobody's happy.''
It will probably take five to seven days before parts of the waterway are reopened, officials said Tuesday night.
Herald researcher Elisabeth Donovan contributed to this report.