The Great Salt Lake dropped 4.2 inches from Sept. 1 to Sept. 15, moving rapidly toward the level at which the state will shut off the pumps that have been pushing water into the west desert.

Under an agreement with the U.S. Air Force, which has a bombing range west of the lake, the pumps will be shut off when the south arm of the lake reaches 4,206.70 feet above mean sea level. It's now at 4,207.05 feet.How soon it will reach the limit is hard to predict, said National Weather Service meteorologist Mike Conger. The six- to 10-day forecast is for cool weather with some precipitation.

As of Thursday, the lake had dropped 2.5 feet from this spring's peak and 2.8 feet from its level this time a year ago.

It is down 4.8 feet from its recorded high of 4,211.85 feet on March 30, 1987, said Mike Remillard, supervisory hydrologic technician for the U.S. Geological Survey, the agency that measures the lake.

Remillard said it's difficult to tell how much of this year's drop in the lake is due to the pumping, because pump operation is erratic - sometimes one pump is operating, sometimes three.

"The biggest factor on the lake at any time is evaporation," he said.