The river provides drinking water for 15 million people in the basin, including residents of Philadelphia and some of its suburbs.
The suit, which was praised by environmental groups but dismissed as frivolous by industry, names the U.S. Army Corps of Engineers and several other federal agencies.
The Army Corps is the federal member of the Delaware River Basin Commission, which governs water withdrawals and water quality in the basin and is poised to adopt regulations for natural gas drilling, possibly before the end of the year.
But the commission itself, whose other members include New York, New Jersey, Pennsylvania, and Delaware - all states with land in the basin - was not named as a defendant.
New York state also has a moratorium on drilling until its own environmental-impact statement is finished.
Clearly, the Attorney General's Office is "looking to get action, looking to get an environmental-impact statement prepared" for the basin, said Ross H. Pifer, a Pennsylvania State University law professor.
"But in the long run, the important issue is what ultimately the DRBC will do," he said. "Will the commission determine that they are going to need additional information before promulgating final regulations? Or will they be satisfied with the information they have already compiled?"
Spokesmen for the DRBC and the Army Corps declined to comment, saying they had just learned of the suit.
Schneiderman contends that the Army Corps and other federal agencies he named fall under the National Environmental Policy Act (NEPA), which requires federal agencies to conduct reviews of actions they take that may cause significant environmental impacts.
But in a May 24 letter to Schneiderman, the Corps' representative on the commission, Brig. Gen. Peter A. DeLuca, said that "the DRBC itself is not a federal agency subject to NEPA, and the mere participation of a federal officer in the DRBC regulatory process does not constitute a federal action.
"We therefore believe that the federal commissioner [on the DRBC] is neither required to produce, nor has the statutory authority to perform, a study under NEPA as part of this process," he added.
"We certainly share Gen. DeLuca's view, and do not believe that litigation is necessary," said Travis Windle, a spokesman for the Marcellus Shale Coalition.
"Frivolous lawsuits and more unnecessary regulatory red tape - which will add no environmental benefit - is not a commonsense solution to address our nation's energy challenges," he said.
Environmental groups said the suit was a vital step and, if successful, would be a game-changer for natural gas drilling in the basin.
Delaware Riverkeeper Maya van Rossum said that the commission "failed to do the basic step in any decision-making process, and that is: Gather accurate information. They're flying blind."
Nevertheless, "nobody is looking to score points with legal tensions here," she said. "People are looking to get the science needed to make a good decision about drilling that affects our environment and our citizens."
In 2009, as drilling spread across Pennsylvania, which is underlain by the gas-rich Marcellus Shale formation, the commission's executive director, Carol Collier, determined that drillers could not proceed in the Delaware basin without DRBC permission.
For that, drilling regulations were needed.
At one point, the DRBC considered doing a cumulative impact analysis, but it had no funds to pay for it. U.S. Rep. Maurice D. Hinchey (D., N.Y.) and others tried to get a $1 million congressional appropriation for this effort in the 2011 federal budget, but they were unsuccessful.
All or portions of seven northeastern Pennsylvania counties that are underlain by shale are in the river basin. There, many economically stressed communities and landowners have welcomed the prospect of natural gas drilling and say that officials who will not allow it are taking away their property rights.
Others cite problems and incidents that have occurred in the rest of the state's shale territory.
On April 19, one day after Schneiderman had sent the Army Corps a letter announcing his intention to sue, a blowout occurred at a natural gas well in Bradford County during a process known as hydraulic fracturing, or fracking, in which several million gallons of water mixed with additives are forced underground at high pressure to break apart the formation and release the gas. Thousands of gallons of water with fracking chemicals flowed into a nearby creek, and seven families were evacuated.
The commission and its staff have said that drilling poses potentially significant effects for the waters of the basin. It has estimated that 15,000 to 18,000 natural gas wells could be developed in the watershed.
The commission proposed regulations in December. By the time the public-comment period closed April 15, more than 58,000 submissions had come in.
The suit, filed in U.S. District Court in New York City, also names the National Park Service, the U.S. Fish and Wildlife Service, and the Environmental Protection Agency, all of which have input into the policy decisions.
Contact staff writer Sandy Bauers at 215-854-5147 or firstname.lastname@example.org. Read her blog at http://go.philly.com/greenspace