Five years and $3 million after Congress ordered safety inspections of each of the nation's 50,000 large dams - inspections that have never been made - President Carter yesterday promised to begin the program.

Carter told his regular, bimonthly news conference that interest in dam safety has been rekindled by Sunday's dam collapse in Toccoa, Ga., which killed 39 and injured 45.

Such rekindled interest has "died down after a few weeks" in the wake of past tragedies, the President said. "I don't intend to let this interest die down . . . I intend to pursue this dam safety inspection now without surcease. It will not be postponed any further."

In the five years since Congress passed the 1972 Federal Dam Inspection Act, the safety problem may have grown worse, according to congressional testimony.

A series of House subcommittee hearings on the issue in March and June of this year showed that:

By the best available government figures, an average of six new dams are started in the United States every working day. But most states do not have adequate programs to make sure they are safe when finished, or that they stay safe as they age.

No one ever knew how many dams there were until the Army Corps of Engineers in response to one section of the 1972 law, conducted a survey and came up with 49,329. The corps admits that figure compiled in 1974, was only about 90 per cent accurate, and there could be a couple of thousand more.

At least 40 per cent of those dams, and perhaps many more, could kill significant numbers of people and damage property if they fail.

Without some sort of national safety inspection program, there is no way to tell how many of these 49,329 existing dams may be deteriorating, unable to withstand the pressures they were designed to begin with.

Only about 5,000 of those dams are under federal control. Most were built by state or local governments, public service agencies or by private companies or individuals.

Less than 18 per cent have been inspected under any federal or state law. And the rate at which they are being built is going up.

According to Dr. Bruce Tschantz, the University of Tennessee's nationally recognized dam safety expert who is advising the White House on the issue, the President's home state of Georgia is one of seven states that have no laws controlling dam construction or maintenance.

The others are Alaska, Missouri, Alabama, Florida, Delware and Hawaii, he said.

Thirty states "appear toaddress the matter of safety in the protection of "while the intent of the remaining 13 is to protect water rights."

But even among the 30 that have some from of safety laws, he told a House subcommittee, many have laws that are inadequate, or insufficient funds to enforce what appear to be adequate laws.

Tschantz concluded the "about half of the states have little or no effective safety control on about a third of this nation dams."

He said that two-thirds of the states which responded to a recent University of Tennessee survey said they could not run effective programs without some form of federal assistance.

In a report issued in June, the General Accounting Office said no dams have been inspected under the 1972 law because the Nixon and Ford administrations decided inspections should be a state responsibility, even though Congress had said otherwise.

A December, 1972, Corps of Engineers request for $5 million to begin the inspections mandated by congress was rejected by the office of management and Budget, the GAO said. A month later OMB issued a policy statement saying inspections "were to be accomplished by the concerned states as part of their normal responsibilities."

A year later the Army informed Congress of that decision. Congress made no attempt to force funds on any administration until after the Teton Dam collapsed as it was being filled for the first time in June, 1976, killing and doing $400 million in damage.

Last July Congress passed a bill giving the Carter administration-15 million it didn't ask for to begin a pilot program of inspections.

That $15 million has been hung up in OMB since. Asked Wednesday why the corps still has not begun inspections, a spokesman said, "We're waiting for guidance from the administration, from OMB."

Carter did not indicate whether he plans to urge the spending of more than the $15 million earmarked for this fiscal year. Corps officials have estimated it would cost $367 million to inspect all the dams if the inspections were carried out over a five-year period.

Carter called in April for the eight federal agencies that build or control dams to review all of their programs related to dam safety.

He said at that time he was taking the action because of the collapse of the Teton Dam. Reports on those reviews were due Oct. 1, and are now expected to go to the White House within two weeks.

Sunday's collapse of the Kelley Barnes Dam near Toccoa indicates what can happen without inspection programs.

The 26-foot-high dam was built privately on private land and was owned by small Toccoa Falls College, a Bible school. The college was downsteam, 800 feet below the dam, and it suffered most of the damage. Most of the 39 deaths were students or others associated with the school.

College officials could not remember any inspections of the dam by trained personnel. Vice President E. R. Hansen said several of his maintenance men walked up to the dam a couple of hours before it burst, looked around, and pronounced it "solid as a rock."

But Tschantz, who visited the site Monday for the White House, said he saw evidence that pine trees had been growing on the dam face, that brush may have covered it and that a 30-inch pipe going through the dam was "supported and surrounded by wooden timbers."

Letting trees and brush grow on a dam in not uncommon, he said, but is "extremely poor maintenance practice." because both tend to obscure signs that there are problems, and if trees die or get blown over, they leave holes which weaken the dam.

Timbers are something "that one doesn't see in well-constructed dams," he said. The one thing you have to avoid is putting material in there that can rot away.