Because last month was dry, not much water flowed down the Potomac River.

Self-evident? Sure. But the Interior Department's Geological Survey makes it official in its monthly review of the flow of fresh water into Chesapeake Bay.

The dry statistics--very dry, indeed--show that the Potomac flow averaged 1.6 billion gallons a day during August, down 40 percent from July and 25 percent below the long-term average for the month, according to Myron Lys, an agency hydrologist.

For all fresh-water tributaries to the Chesapeake, Lys said, the inflow of 10.9 billion gallons daily was the lowest August inflow since 1968. But don't panic: The low record was set in August 1966 with 6 billion gallons daily.

Back to the Potomac, whose waters most of us use to maintain sanitation and hydration. The Washington area communities took an average of 384 million gallons a day from the river for domestic use, less than a quarter of the flow.

And what of groundwater levels, that figure so importantly to area farmers?

They're dropping.

At what is described as a "key index well" near Fairland, in northeastern Montgomery County, the level dropped below the long-term average for the first time in five months. The water table was 13.9 feet below the land surface, six inches below the long-term August level.