Officials with Seattle Public Utilities are investigating what caused a siren linked to the Tolt Dam emergency warning system to mistakenly trigger earlier this week.

On two separate occasions Monday (June 19), sirens connected to the Tolt River Dam Early Warning System went off and broadcast an evacuation message, the City of Carnation said in a statement Tuesday. The system is suppose to alert residents of failure on the Tolt Dam.

Around 10:18 a.m. Monday, a siren near town and a siren near the Tolt River’s edge went off for about ten minutes, said Alex Chen, a director with Seattle Public Utilities (SPU). The siren near rivers edge then went off a second time at 5:25 p.m, he said.

Chen told the Carnation City Council Tuesday they are unsure what caused the siren to go off, but are investigating the incident and expect to have a report to the city soon. The dam remains safe, he said.

“We don’t have an answer other than we take it very seriously,” he said. “We know the proper functioning of that system is critical to the public’s trust.”

The dam and its reservoir are owned by the city of Seattle and provide drinking water to over 1.5 million Seattle-area residents. Carnation sits just 16 miles downstream. If the dam were to fail – which SPU stresses is unlikely – it could flood the city.

SPU monitors the dam 24/7 and its automated alert system is designed to notify residents in the event of a failure. The alarms are tested by SPU each Wednesday.

In a phone interview, Carnation City Manager Ana Cortez said the city will release more information to the public once SPU concludes its investigation.

“We are confident given the caliber of the individuals at SPU that we’ll get to the bottom of this,” she said. “The good news is the dam is fine.”

The alert Monday lasted only a few minutes, and compared to a similar erroneous alert that happened in July 2020, was much tamer, Cortez said. In 2020, the dam’s warning system went off for 40 minutes, prompting residents to evacuate.

SPU is in the process of overhauling the current alert system, Chen said. Crews are working to phase in a new system that they say will improve reliability. The old system is aging and difficult to maintain, according to SPU.

Each Wednesday, SPU has been testing both the new and old system, ensuring they are functioning as it attempts to fully transition to the new system, Chen said.

“You hear two sets of alarms every Wednesday,” Chen said, adding they plan to do more diagnostics during their regular Wednesday test this week. “Once we have all those diagnostics in hand we will be sending out the analysis.”