About 800 mostly small fish were found dead along Mary's Creek in Benbrook, near a sewer line that spilled 105,000 gallons of raw sewage, the Fort Worth Water Department said Friday.
The creek is polluted and people should stay away for at least a week, said water department spokeswoman Mary Gugliuzza.
The city declared an emergency so a contractor could be hired immediately.
"This is an emergency," Gugliuzza said. "We need to get the sewer pipe fixed."
The break was discovered on Wednesday and was apparently caused by recent rains, which eroded part of the ground underneath and the pipe gave way, she said.
Crews have plugged the broken line and diverted the wastewater temporarily back into the city sewer system.
"We've added sandbags across here to further dam up the flow, and we're trying to capture as much of this wastewater as we can," Gugliuzza said.
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Tests have shown the sewage has not created a problem downstream, where Mary's Creek feeds into the Trinity River.
"The E. coli counts where the creek hits the Trinity are actually below the recreational limit, so it's showing there's no negative impact," Gugliuzza said.
It could take weeks to finish the repairs to the sewer line and clean up the mess left behind.
The project is expected to cost about $300,000.
Meanwhile, the Texas Commission on Environmental Quality said it was monitoring the situation.
"The TCEQ is still evaluating the extent of the sanitary sewer discharge," spokesman Brian McGovern said in an email.
