The 60-inch pipe broke Saturday afternoon, and the San Francisco Public Utilities Commission (SFPUC) has been working to shut it down completely ever since.
The San Mateo Creek runs about five miles from the Crystal Springs Reservoir out to the San Francisco Bay.
The burst pipe sent water that's safe for humans but not safe for fish into a tiny slice of habitat that until now has survived against the odds in heavily populated suburbia.
The thousands of gallons that flowed into the San Mateo Creek killed an unknown number of trout, including possibly the endangered steelhead species.
The chlorinated drinking water is now being de-chlorinated.
Stephen Rogers lives along the creek and noticed the dead fish Sunday morning. "And at the bottom of the pools, all the fish were dead," he said.
Some of the fish were lying on the bottom, some were floating on the top and some were on stream banks, he said.
Rogers has more than a passing interest in the trout. He's on the board of an environmental group called California Trout. The group is devoted to protecting and restoring native species in the state.
He is upset and worried it could take years to restore the habitat -- if possible at all. "This was historically a steelhead stream. It's not good," he said.
"We don't like to kill fish. We don't want to kill fish. So we got to find a way to keep this buttoned up," said Steven Ritchie, the SFPUC's assistant general manager of the water enterprise .
The SFPUC says it doesn't know yet what caused the pipe to break, but age certainly played a role. It was 75 to 80 years old and scheduled for replacement.
A handful of environmental agencies are investigating for possible penalties against the PUC.
The regional Water Quality Control Board and the National Marine Fisheries Service have been informed, Ritchie said. "[It's] their job to determine if penalties should be assessed, and they'll do their job."
The steelhead are categorized as threatened under the Endangered Species Act. There'll have to be some DNA testing to determine exactly which species were killed in the creek because of this spill.