Taum Sauk price tag: $450 million

AmerenUE on Friday said it will cost $450 million to rebuild the mountaintop reservoir that feeds the Taum Sauk hydroelectric plant — about nine times the price of the original structure a half-century ago.

The reservoir is expected to be complete by late 2009, and the utility said it expects insurance to cover substantially all of the costs. The utility is prohibited from billing customers for any rebuilding costs.

The 400-megawatt Taum Sauk plant has been idle since Dec. 14, 2005, when a 650-foot section of the wall encircling the giant, kidney-shaped reservoir on Proffitt Mountain collapsed, releasing more than a billion gallons of water that scoured the mountainside and Johnson’s Shut-Ins State Park and injured the family of the park superintendent.

A federal investigation showed the collapse was triggered when water flowed over the top of the reservoir and eroded a rockfill dam. Water-level sensors that were supposed to prevent the reservoir from overflowing had been moved high enough to render them ineffective.

Ameren paid $15 million in penalties under a settlement with the Federal Energy Regulatory Commission in October 2006. And in November, the utility agreed to a $177 million settlement of a lawsuit by Missouri Attorney General Jay Nixon.

Federal regulators have approved Ameren’s rebuilding plans, and work on the project began late last year.

The new structure will be made entirely of concrete, not pieces of rock from the mountain used in the original. It will include a spillway and a video monitoring system to ensure there isn’t a repeat of the collapse, company officials have said.