Officials Warn of 1,000 Fish Found Dead in L.A. River : Health: Lab workers fear people will eat them. A mysterious brown chemical is found in the water in the Sepulveda Dam area.

About 1,000 dead fish floated to the surface of the Los Angeles River in the Sepulveda Dam area, apparently killed by a mysterious brown substance illegally dumped in the water, and Los Angeles city officials worry that some people may be harvesting them to eat.

“We’re concerned because some of our lab workers spotted people picking up the dead fish and taking them with them,” Anna Sklar, spokeswoman for the Bureau of Sanitation, said Thursday.

“We want to alert as many people as possible not to eat the fish,” Sklar said. “We don’t know yet what caused the death of the fish.”

Many people regularly fish and swim in the river, and transients who live in the surrounding park have been seen drinking the water, said Lowell Flannery, spokesman for the Army Corps of Engineers, which owns the dam and the land around it.

Even “in the best of times, when there is no fish kill . . . we wouldn’t encourage swimming or fishing in there,” he said. “There is considerable pollution in the water.

“You’re not in pristine mountains with crystal water,” he said. “It’s urban runoff,” including the outpouring from storm drains throughout the western San Fernando Valley.

The dam in Encino controls the flow of runoff from the bowl-shaped Valley into the river, which then runs through the Glendale-Griffith Park area and downtown Los Angeles to the ocean. The dam was constructed and the river was lined with concrete for most of its course as flood-control measures many years ago.

The river around the dam is home to several species of fish, however, including talapia and minnows, officials said.

A dam operator noticed the dead fish Wednesday morning and notified the sanitation bureau, which began a series of tests on the water, Sklar said.

Laboratory workers found a brown, foamy material in the river just north of Balboa Boulevard about half a mile from the dam, but have not determined what the chemical is, Sklar said.

“The one thing we know is the dissolved oxygen level was low and the fish probably suffocated,” Sklar said. A series of tests was being conducted, including one for petroleum-based products, because of the appearance of the water, Sklar said.

The size of the spill was not known.

The Tillman Reclamation Plant, which processes sewage and releases the cleaned-up effluent into the river, is about a quarter mile upstream from the spill. Initially, there was a suspicion that sewage killed the fish, but “the water reclamation plant has not had any type of spills,” Sklar said. “It’s definitely not sewage. We ruled that out early on.”

By Thursday, most of the dead fish had drifted away, but some could still be seen floating along the river and decaying on the riverbanks.

“People should also exercise precaution about allowing children or pets in the water,” Flannery said. “No internal consumption should be considered at any time.”

The results of tests on the water could come back as early as Friday, Sklar said.