Last month, the Interior Department's Geological Survey reports, the Potomac River was on a real high. Literally.

The river's rain-swollen flow averaged 79 percent above normal and helped produce the third highest freshwater flow for a single month into the Chesapeake Bay in at least 30 years, according to Geological Survey hydrologist Myron Lys, who is stationed in the Baltimore suburb of Towson.

Total freshwater inflow to the bay--from streams with their headwaters in New York, Pennsylvania, Delaware, Maryland, West Virginia, Virginia and the District of Columbia--averaged 96 billion gallons a day, about 56 percent above average. (The biggest flow on record for the month of May was set in 1978, with a daily average of 118 billion gallons.)

In the Washington area, the Potomac flow averaged about 16 billion gallons daily, 79 percent above normal. The range was from 7 billion gallons on May 7 to 32 billion on May 19.

Ground water levels remained about two feet above normal at the Geological Survey's test well at Fairland, in northeastern Montgomery County. That's good news for growers and gardeners.

An interesting statistic: most of the freshwater flow into the bay (56 percent) came from the Susquehanna River. The Potomac River provided 21 percent and the James River, which flows through Richmond, provided 10 percent. All other streams, including the Rappahannock, produced the remaining 13 percent.