ASHINGTON, Dec. 9 — The Supreme Court handed Virginia a decisive victory on Tuesday in that state's battle with Maryland over the right to draw drinking water from the Potomac River, which forms most of the boundary between the two states.

Maryland's undisputed ownership of the entire bed of the river, to the low-water mark on the Virginia shore, does not give it the authority to regulate withdrawals of water that do not interfere with navigation, Chief Justice William H. Rehnquist wrote for the court. The vote was 7 to 2, with Justices John Paul Stevens and Anthony M. Kennedy dissenting.

Three years ago, Virginia invoked the Supreme Court's so-called original jurisdiction to hear disputes between states after Maryland's Department of the Environment asserted the authority to block Virginia's Fairfax County Water Authority from building a new intake pipe to extend 725 feet from the Virginia shore, near the midpoint of the Potomac channel.

In Virginia's view, the issue was one of local protectionism, because Fairfax County, its water needs fueled by economic development, competes for business with Montgomery County, Md., directly across the river.

The justices referred the case, Virginia v. Maryland, No. 129 Original, to a special master, Ralph I. Lancaster of Portland, Me. He recommended a ruling for Virginia. The court heard arguments in Maryland's appeal in October.

Chief Justice Rehnquist based his decision on two documents, a 1785 compact between the two states and an arbitration decision from 1877 known as the Black-Jenkins Award. The compact gave the citizens of each state the right to use their respective shores, including "the privilege of making and carrying out wharves and other improvements."

The Black-Jenkins Award for the first time fixed the long-disputed boundary between the two states, giving Virginia "a right to such use of the river beyond the line of low-water mark as may be necessary to the full enjoyment" of its rights as a shoreline landowner.

"The arbitrators did not differentiate between Virginia's dominion over the soil and her right to construct improvements beyond low-water mark," Chief Justice Rehnquist wrote. Accordingly, he said, Virginia had "sovereign authority, free from regulation by Maryland" to take water from the river.

The ruling did not appear to reflect any sectionalism on the part of the justices. Both dissenters, who voted for Maryland, live in Virginia, as do the chief justice and two others in the majority, Justices Antonin Scalia and Clarence Thomas. Among the others in the majority, Justice Sandra Day O'Connor lives in Maryland and Justices David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer live in the District of Columbia.