To control flooding along the popuious banks of Boston's Charles River, the old Army Corps of Engineers might have built an expensive dam, or dredged a deeper channel. Today's Corps is buying up wetlands to be preserved as a natural sponge, free of development.

In northern Louisiana, the Corps is a partner in the fight for acquisition o some 50,000 acres of Tensas Basin bottomland as a wildlife refuge to offset some of the damage done by its own past construction there.

Canada geese winter on a manmade lake near Benton, Ill., and children play on a giant "grass waterway" which doubles for baseball and flood control in Scottsdale, Ariz., both under the supervision of the Corps.

Even some of the engineers' worst enemies now are conceding that they have worked the ultimate bureaucratic miracle -- they have changed.

First mustered the day before the battle of Bunker Hill, and motley band of engineers worked through the night building earthwork fortifications for the Continental Army. In the 200 years since, the Corps has built itself into a classic bureaucratic juggernaut. Powdered by enormous budget authority from allies in Congress, the Corps has been able to pursue its civil works mission -- development of the nation's water resources -- often independent of either military or executive branch control.

The Corps is still in the thick of many battles, perennially linked with such headline grabbers as the billion-dollar-plus Tennessee Tombigbee Waterway, under attack as one of the biggest boondoggles in history.

But now, like a mongoose blowing kisses at a cobra, environmentalists who have been locked in combat with the Corps for years are making comments like this one from Patrick Parenteau of the National Wildlife Federation: "It's true, the Corps in the last few years has come a long way. They are buying wetlands instead of building dams, doing better at planning to minimize habitat losses for fish and wildlife, and they are doing a little better in screaming out uneconomic projects and generally thinking of new approaches."

W. T. Olds of the Interior Department's Fish and Wildlife Service, a leading federal advocate of environmental interests, says, "The Corps has made considerable improvement . . . For instance, on a number of older projects they built, they are going back and taking a look to see if they can't offset some of the damage done to fish and wildlife. We call it 'retorfitting.' One big item we are really proud of them about is the Tensas project in Louisisana."

Olds also praised the Corps for administering its regulatory program -- issuing development permits, for example -- with "much more consideration of ecological concerns."

A major turning point, bu all accounts, came in the mid-'70s when the Corps astonished many of its adversaries by denying a development permit to the Deltona Corp. for a project on Marco Island near Naples, Fla. The company had planned a waterfront community that would have destroyed mangrove swamps and affected the ecology of the area. The Corps' decision to deny them certain water access was, as one official put it, "very costly" to the developer.

The Corps, self-billed as the world's largest engineering organization (and, many believe, its most able), had completed nearly 3,400 projects by the mid-'70s. Appropriations for its 200 works in progress currently total $3 billion, with billions more required to complete them.

The engine of tis institutional change of heart was public pressure expressed in a new law passed by Congress, according a retired Army Gen. John W. Morris, now in private industry, who reportedly was instrumental in the shift while he was the Corps' chief engineer during the 1970s. The 1969 National Environmental Policy Act (NEPA) put a new emphasis on environmental concerns and by 1974 dramatically altered a system which had given the Corps funds for dams but had denied it money for other, sometimes more desirable, solutions.

But Morris' account, the Corps' effort to respond to the new law was the "appropriate and legal" thing to do. Actually, according to a Brookings Institution study, most agencies "paid lip service to environmentalism and citizen involvement" in the wake of the new law but, in true bureaucratic tradition, changed little. The Corps was the exception. "The agency seemed to be making a conscious and serious effort to accommodate itself to the spirit of the environmental movement as well as to the letter of the law," a 1979 study, "Can Organizations Change?", by Daniel Mazmanian and Jeanne Nienaber, said.

The Corps suffered considerable "command and management problems" in its effort to change, Morris said. Still, "the decision was made to try to weave environmental considerations into every step [of a project], rather than waiting until the eleventh hour and then conducting a review."

The Corps' ability to make the kind of massive readjustments that other bureaucracies would not or could not achieve is due in part to having the military's responsiveness, said Maj. Gen. E. R. Heiberg III, director of the Corps Civil Works Division. Its organizational advantages include a high degree of decentralization and frequent changes of leadership out in the field, "where the rubber meets the road."

However, the Brookings study warend that the Corps' reformation might falter in the absence of continued public pressure and questioned whether the Corps would be able to maintain its new effort "if development interests are given freen rein."

Some adversaries contend the Corps fall short of its stated environmental goals in many instances. In any case, they say, many of their fights today with the Corps are not over new projects, but over old "boondoggles" funded years ago that refuse to die.

Officials of the Corps, for their part, note that some environmentalists have softened their former "kneejerk negative" approach to any development and are learning a more positive give and take.

Both sides made clear that, despite the improvements, they are still at odds in numerous areas.

Some environmentalists credit James G. Watt, President Reagan's controversial secretary of the Interior, with quickening the liaison between the Corps and its arch foes because of what they see as his efforts to make a power grab for water resource development authority. This, they say, threatens the Corps turf as well as environmental interests.

Through his Cabinet council on natural resources and environment (an arm of Reagan's "Cabinet government" structure), according to Parenteau and others, Watt is attempting to push through certain water projects under the Interior Department's Bureau of Reclamation, a traditional rival of the Corps.

It's true that "in the past the Corps has lusted after a project that [the bureau] gets," said Gen. Heiberg. But he argued that this tension is just "good healthy competition" and that Watt has satisfied the Corps' early concerns that the Army will have "a chance to weigh in on water policy."

Under the Reagan administration, the political pendulum has swung back in the direction of developmental interests. Heiberg observed. "But it has swung back to a balance [between environmental and developmental], not al lthe way back to development."

He contrasted the new administration favorably with the Carter administration and praised it for managing to give the Corps a decisive go ahead, without "agonizing" or having to go all the way up to the president, on some controversial projects -- such as Manhattan's Westway and the Columbia Dam in Tennessee. He said they likely would still be "hung up" today if the Carter administration had remained.

"It was just time to get off the dime," the general remarked.