A disaster exercise involving hundreds of emergency responders and volunteer victims will be staged on Thursday afternoon on the St. Croix River in Stillwater.

The training scenario will involve a riverboat accident with victims playing wedding guests on board. But exactly what happens to the boat at 4:30 p.m. remains a secret to preserve some spontaneity, said Deb Paige, emergency management director for Washington County.

As many as 300 people -- at least 90 of them victims -- will play out a major disaster involving police and fire agencies, ambulances and dive teams.

"Injured" victims will be taken to hospitals on both sides of the river, and non-injured victims will be taken to a family assistance center to be reunited with loved ones, Paige said.

The exercise will involve a riverboat owned by the St. Croix Boat and Packet Co., a popular river concession just south of the Stillwater Lift Bridge.

The exercise is funded with Homeland Security grants, Paige said.

"This exercise helps us understand what we do if this really happens," she said.

And afterward? "What went right, what didn't go so well and an improvement plan."

Other participating agencies include the Washington County Sheriff's Office, Stillwater fire and police, Lakeview Hospital, American Red Cross and Medical Reserve Corps.

KEVIN GILES