That fundamental question, U.S. District Court Judge Paul A. Magnuson said, could render other legal questions in the court battle "obsolete" and allow it to be resolved as early as January. He also said an appeals court ruling earlier this year that invalidated some of Georgia's rights to Lanier would "undoubtedly affect" the litigation.
Magnuson's decision came in response to a motion from Florida and Alabama in a nearly 20-year court fight over the Apalachicola-Chattahoochee-Flint river basin. The case is being heard in a U.S. District Court in Jacksonville.
Florida and Alabama say Georgia is withholding too much water from Lanier, drying up river flows into their states that support power plants, commercial fisheries and industrial users like paper mills. The case also involves claims that the lake withdrawals are threatening endangered mussels and sturgeon in the rivers.
Georgia argues it isn't getting enough water from the reservoir. As a record drought threatened drinking water supplies last year, it pressed the federal government for more.
The governors of the three states tried to work out a compromise earlier this year but failed.
In February, Alabama and Florida won a major victory in a related case when the U.S. Court of Appeals in Washington threw out an agreement that would have allowed Georgia to expand its use of Lanier for future water supplies. The court said Georgia and the Army Corps of Engineers needed congressional approval to make such a significant change to operations at Lanier, which was originally built for hydropower.
Alabama and Florida have since argued that the decision bolsters their argument in the lower court case that Georgia has no legal right even to its current withdrawals from Lanier.
Georgia has sought to downplay the appeals court decision. But in his ruling, Magnuson repeatedly cited its impact.
"That court has now ruled, and the issues addressed in that decision will undoubtedly affect this litigation," he wrote.
Meanwhile, an attorney for Georgia said the state would appeal to the U.S. Supreme Court on Wednesday to get the D.C. court's ruling thrown out. The attorney, Todd Silliman, said the appeal was planned before Magnuson issued his ruling.
A spokesman for Georgia Gov. Sonny Perdue did not immediately return a call seeking comment.
Alabama Gov. Bob Riley issued a statement applauding Magnuson's ruling.
"Today's court order means that the end is in sight," Riley said. "The federal appellate court in Washington has already validated Alabama's position that Atlanta's future water supply plans violated federal law. Alabama believes that Atlanta's current withdrawals also violate federal law, and we are pleased that the federal court has placed that issue on a fast-track for resolution."