The Potomac River, recharged by heavy rains in July and August, completed the so-called "water year" ending Sept. 30 at about 60 per cent above the normal flow rate, the U.S. Geological Survey reported yesterday.

The river, which provides drinking water for the District of Columbia and portions of suburban Maryland Virginia, averaged 10.9 billion gallons a day at Little Falls. The normal annual flow rate is about 6.8 billions gallons a day.

The "water year" as defined by the Geological Survey, begins on Oct. 1 and ends Sept. 30, corresponding roughly with the growing season.

The individual daily flow of the Potomac in the most recent year ranged from a maximum of 90.7 billion gallons on March 17 to a minimum of 1.1 billion gallons last Oct. 7, according to the survey.

Diversions of water for municipal and county use never exceeded 398 million gallons a day. With total flow always above one million gallons a day, the water supply was never threatened, as has happened on several occasions in the past two decades.

Higher than normal rainfall throughout the Potomac basin kept the river at healthy levels during the entire year.

A total of 32.45 inches of precipitation has been recorded in the first nine months of 1978 at Washington National Airport the official measuring point for the National Weather Service in this area. Normal precipitation in the first nine months is 30.29 inches.

The rainfall, as well as generous snows last winter, also gave the local ground water levels a boost. At the end of September, ground water observed at a key observed well stood at 13.1 feet below the land surface, about 3 feet above the long time average for this time of year, the survey said.