A federal judge ruled against Georgia in the state’s water dispute with Alabama and Florida, deciding that Atlanta must stop withdrawing water from a massive federal reservoir within three years unless if can get approval from Congress. The judge, Paul A. Magnuson of Federal District Court, said that Lake Lanier had not been built for water supply and that the state’s withdrawals were illegal. Judge Magnuson acknowledged that it would be impossible to stop using the lake immediately because it is metro Atlanta’s main water supply. But he said that if the state could not get Congressional permission within three years, the withdrawals must end. The case involves a 2003 water-sharing agreement with the Army Corps of Engineers that would have allowed Georgia to take more water from Lake Lanier for its drinking supply. Florida and Alabama objected, saying that the lake had been initially built for hydropower and that providing water to Georgia was not an authorized use.