''We have, in light of all that's occurred, implemented security measures we don't discuss in terms of specifics,'' said Jim Parker, a spokesman for the Army Corps of Engineers, which manages the 70,000-acre Thurmond Lake.

One immediate impact is the cessation of tours inside the mile-long earth and concrete dam.

''The dam and powerhouse are no longer open to visitors, and that is until further notice,'' Mr. Parker said. ''The dams at lakes Hartwell and Russell are under the same precautions.''

Although current restrictions are ''until further notice,'' Mr. Parker said it is possible tours might be curtailed permanently.

A successful sabotage of any of the three Corps dams on the Savannah River could cause failure of any dams downstream and put much of downtown Augusta under as much as 32 feet of water.

Although access to sensitive areas within the dams has become more restrictive, recreational areas will remain open.

The Corps' precautions mirror those taken by other potential terrorist targets.

PCS Nitrogen, for example, has tightened its already-strict security system in part because one of its primary products - ammonium nitrate - is an ingredient used in explosives.

''We even discontinued shipment of ammonium nitrate last week for several days while we were at a point in time when things were actively happening,'' company spokesman Pete Brodie said.

Although valuable as a fertilizer, ammonium nitrate can be mixed with other compounds to create large bombs, such as the one used in the Oklahoma City bombing. PCS is the major East Coast manufacturer.

''We're exercising far more stringent controls over that product,'' Mr. Brodie said. ''We're even at a point where we're photographing truck drivers.''

Reach Robert Pavey at (706) 868-1222, Ext. 119, or rpavey@augustachronicle.com.