Thurmond Lake fell to its lowest level since 2002 this week, triggering another set of restrictions on how much water the Army Corps of Engineers can release from the reservoir into the Savannah River.

As if the drought weren't creating enough problems, fish kills caused by fluctuating oxygen levels upstream halted hydropower generation several days in a row. About 80,000 fish were killed.

The fish -- mostly blueback herring -- prefer cold, oxygenated water and typically move around in search of preferred habitat. When that habitat coincides with turbine intakes 70 feet below the lake's surface, fish kills occur and hydropower generation must be halted.

An estimated 70,486 fish were sucked through the turbines last week, bringing hydropower production to a halt for seven of the past 11 days, said Jamie Sykes, the corps' district fisheries biologist.

The river below the dam is being monitored for additional fish kills, which could cause the dam's operators to cease generation if the rate exceeds 5,000 fish per hour.

Blueback herring are a preferred forage fish for open-water predators, including hybrid and striped bass, and past estimates have placed the lake's herring population at 68 million.

Comparable fish kills occurred at the lake during similarly hot weather in 2004, 2006 and 2007.

Although declining water levels did not directly cause the fish kills, the falling pool creates additional rules the corps must follow in managing the lake.

Just after 2 p.m. Monday, the pool level dipped to 316 feet above sea level -- or 14 feet below full pool -- said corps spokesman Billy Birdwell.

The milestone typically requires the corps to limit its releases through the dam to no more than 3,600 cubic feet per second on a weekly average, but the corps has been holding releases to that figure for several months on a voluntary basis, he said.

Falling to the 316 mark also triggers Level 3 of the corps' drought management plan, but the public will see no real changes because the releases are already being held to a minimum.

The last time the lake fell below 316 was Sept. 6, 2002. Before that drought, the lake had not fallen so low since Feb. 21, 1989, when the pool dwindled to 312.91 feet above sea level.