Both those who reported the mysterious fluid and the personnel who responded were concerned that it could be antifreeze, which would be devastating to the wildlife in the creek.

But a test revealed the substance to be nothing more harmful than food coloring and was later identified as a dye used by the Los Gatos Parks and Public Works Department to test storm-drain water flow.

A Santa Clara County Fire Department hazardous-materials team, several Los Gatos­Monte Sereno police officers, county park rangers and a water-district employee arrived at the creek area between Bachman and Almendra avenues after someone reported seeing the unknown substance on June 25.

Parks and Public Works Director John Curtis said a department crew was performing a dye check, in which a harmless coloring agent is placed in a drainage line to track the flow of water, on Massol Avenue. Some concerned passersby spotted the dye.

"There were some people walking along the Los Gatos Creek Trail, and they stopped in our engineering office and made the point that there was something in the creek," Curtis said.

But Curtis said department officials did not know the field crew had used the dye in that area, so the call was treated as a potential hazard.

Santa Clara Valley Water District spokesman Mike DiMarco said any unknown substance in the creek must be treated with caution due to sensitive species of fish.

"This was a bright green-colored liquid that looked like antifreeze," he said. "That was our fear. We have steelhead trout and Chinook salmon in the creek."

DiMarco said that a chemist was not available until midnight that day to test the substance. Even after that, other agencies were not immediately notified of those results.

Neither police Sgt. Michael Barbieri nor county Hazardous Materials Specialist John Justice knew what the substance was as of June 29.

Asked if the parks and public works department normally notifies other town agencies when it will be doing dye checks, Curtis said they are done too spontaneously to give notice. But he said he might try to do so in the future to avoid further confusion.

"We're not going to notify everybody under the sun when we do that, but we can do some notifying," he said.

He added that the fire department did not ask parks and public works personnel if they had been doing such testing, but that they responded appropriately.

"I think what is valid is that the hazardous-materials people do check what's going on," Curtis said.