ST. LOUIS -- The Missouri Department of Natural Resources has billed Ameren Corp. nearly $495,000 to pay for the agency's oversight of clean-up efforts resulting from the collapse of Ameren's Taum Sauk Reservoir. The reservoir collapsed Dec. 14, sending more than 1 billion gallons of water rushing through Johnson's Shut-Ins State Park.

The DNR has been supervising clean-up efforts since the collapse, and will bill Ameren every three months to compensate the agency for staff time and materials, said deputy director Kurt Schaefer.

Schaefer said the $495,000 bill was sent to Ameren late Thursday and will cover DNR's costs for the first three months of this year. He said the bill will likely be the largest one Ameren will have to pay for DNR oversight because work efforts were most intense immediately after the reservoir collapse.

The DNR's quarterly bills do not cover the total costs of damages or fines, Schaefer said. Missouri Attorney General Jay Nixon is preparing to sue Ameren over the reservoir breach.