It's easy to tell when Thurmond Lake needs more water, but a newly redesigned federal project will increase a less visible commodity: dissolved oxygen.

It will also improve fishing in the lower end of the lake.

Construction began this summer on a cryogenic oxygen plant that will pump 20 to 100 tons per day of liquid oxygen through seven miles of perforated pipes submerged 80 to 90 feet below the lake's surface.

Although the project will be almost invisible to visitors, its impact will come in the form of a swath of highly oxygenated water that will lure and hold sportfish -- and fishermen -- during the hot summer months.

Deryck Solomon, the manager of the Army Corps of Engineers project, said the first of three construction phases is under way now and involves road improvements to accommodate the frequent truck traffic required to bring oxygen to the plant.

"The road work is going well," he said. "Weather permits, we hope to finish maybe before Thanksgiving."

The site off Gilchrist Road near Modoc, S.C., will also have a permanent storage facility and cryogenic oxygen plant. The above-ground portion is the project's second phase, followed by the final phase that includes the submerged distribution system.

The completed facility is expected to go online in the fall of 2010, Mr. Solomon said.

The project's price tag, estimated at $6 million in 2006, has risen to $11.3 million. The higher costs are mainly because of an improved design sanctioned by the Tennessee Valley Authority, which has built several of the oxygen systems at its hydropower projects.

"The Tennessee Valley Authority is building the underwater portion," said corps spokesman Billy Birdwell. "They have improved the design, which has driven up the costs."

The corps agreed to build the oxygen system as part of a 2002 settlement to a lawsuit over fish kills caused by Russell Dam's reversible turbines.

Russell's four reversible turbines, which can pump water from Thurmond Lake back into Russell for reuse in hydropower generation, have been shown to raise water temperature if operated all at once during warm weather months.

The oxygen system will create an ideal habitat -- at the lower end of the lake -- for striped bass that normally congregate at the Thurmond headwaters below Russell Dam.

As part of the corps' 2002 operating agreement, only two of the four reversible units can be operated during warmer months to avoid heating up the tailrace area and disrupting striper activities. Once the oxygen system is active on the lower lake, that restriction will be lifted.

Once online, the project is expected to increase dissolved oxygen levels by up to 3 parts per million along a five-mile swath from Modoc downstream to the face of Thurmond Dam.

Current oxygen levels in that area can fall to as little as 1 part per million during warm weather.