The break of a 48-inch water main under the Potomac River Friday was the cumulative result of erosion from storms that disturbed the river bed, an Arlington County official said yesterday. He said countywide emergency water restrictions adopted this weekend may be lifted in a month to six weeks -- earlier than was at first expected.

A diver who found a six-foot-long hole in the pipe Saturday evening attributed the rupture to the scouring action of rock and dirt that had eroded the concrete shell. The break cut off water to the eastern part of the county for five hours Friday morning.

County Public Works Director Henry S. Hulme Jr. said that while he could not determine when the pipe had been weakened, "Generally, it takes a heavy storm to cause this much erosion."

Damage to the 20-year-old pipe was done by "the same kind of erosion action that would take place if you continually grind away at something," he said. He said it will be repaired by replacing two 16-foot sections.

The diver hired by the county found the hole 135 feet from the Virginia shore near Chain Bridge around 7 p.m. Saturday after crawling through the pipe, which is buried about six feet under the river bed, Hulme said.

Hulme, the county's top water official, said that if the county can stockpile water in its storage facilities, emergency restrictions on most outdoor water use may be lifted sooner than the two months predicted earlier. The county gets about 25 million gallons of water daily from the Army Corps of Engineers' Dalecarlia Water Treatment Plant in Washington.

"We may be able to lessen the restrictions if we are able to replenish our supply overnight, when demand is low," Hulme said. The county is now receiving about 85 percent of the water it needs, using three 20-inch pipes suspended below Chain Bridge, he said.

County Board Member John G. Milliken said yesterday that he had been unaware that the emergency restrictions could be lifted sooner than expected, but he added that Hulme "has information we don't have yet." Board Chairwoman Mary Margaret Whipple could not be reached for comment yesterday.

The diver, retained by the county from a Stevensville, Md., firm, swam through the pipe twice before confirming the size and location of the hole in the top of the pipe, Hulme said. "It blew out almost like a tire," Hulme said Saturday night, shortly after the hole was discovered.

Hulme said that the county should have enough information by the end of this week to solicit bids on repairs, which "shouldn't take more than two or three weeks."

He said he did not believe the water main was defective. A spokesman for the G.H.A. Lockjoint Co., of Perryville, Md., which made the pipe, could not be reached for comment yesterday.