In a series of little-noticed votes in the weeks before its recess, Congress moved systematically to shred the new national water policy that President Carter sent it with great fanfare earlier this year.

At different levels, there were these developments:

House and Senate conferees, inviting a veto after Carter and Congress return, agreed on a $10.1 billion public works appropriations bill that includes a number of dam projects the president thought he killed in his first water fight with the legislators last year.

In separate authorization bills, the two houses are challenging the president even further. He wanted to make state and local governments and users pay more of the cost of water projects, and the federal government less, to encourage conservation. The bills would do the opposite.

He wanted to tighten the rules for the cost-benefit studies according to which projects are said to be justified or not. The bill would loosen those rules.

The bills would kill funding the agency Carter wanted to oversee his new policy, while directing him to hire 2,300 bureaucrats to oversee traditional projects.

Variously, the House and Senate bills increase, rather than decrease, the federal share of dredging projects; waive negative benefit-cost rations in some cases; authorize projects not yet studied by the Army Corps of Engineers; thwart Carter's proposal to require the states to share in the cost of water projects.

A series of resolutions and hearings in the House and Senate is seen as another gambit to reshape or flatly overturn Carter's plan to lay down a more consistent, more cost-conscious national water resources program.

Notwithstanding the supposed public revulsion toward more federal spending, waste and bureaucracy-building, Congress seems to have gone out of its way to draw a wide line in the sand in front of Carter.

This confrontation is over who - Congress, the administration or both - is going to decide how much is spent, where and by whom on dams and flood-control projects - the traditional federal pork barrel.

A veto of the appropriations bill, and possibly the authorization bill, seems certain after Congress returns from its Labor Day break.

Also certain are fireworks at least as heated as last year's when the president stunned Congress by nominating 18 dam projects for termination on the ground that they were wasteful, unneeded or environmentally damaging.

The storm that ensued after Carter attempted to trample on projects supported by influentional legislators ended in a compromise of sorts.

Some projects stayed in the budget; some were removed. But the congressional uneasiness continued, abetted by growing western concerns over water needs, as the White House worked out details of the policy proposed in June.

Carter again suggested at a press conference two weeks ago that he would veto the appropriations bill, lamenting that he had not vetoed last year's version.

And for months the administration has bee sending signals that the authorization bill is a veto candidate. It will emerge from Congress with lower waterway user fees than Carter wanted and full of major liberalizations of traditional project-authorizing policies.

Environmentalists, led by the National Wildlife Federation and the Environmental Policy Center, think vetoes are essential first steps if Carter is to have any success in establishing the water policy he has laid out.

"He has made the new water policy a centerpiece of his domestic legislation," said Brent Blackwelder of the EPC. "Carter will be regarded as the world's biggest pushover if he doesn't veto these bills.

"It was handled poorly last year." Carter had the reputation that he was going to take on Congress and attack the pork barrel. But he was naive - he thought he got half of the projects he opposed cut from the budget, but here they are again this year."

Blackwelder and other environmentalists see Congress engaged in a sort of political game this year - inviting a veto by passing legislation that goes far beyond the acceptable, but knowing that compromise in the end will leave intact the legislators' bottomline wants.

The authorization bill, passed by the Senate but awaiting a final House vote, illustrates the new lengths to which the legislators have gone.

Between the two versions, more than 120 new projects are authorized many with no knowledge of final costs. Three dozen Corps of Engineers projects have been approved without even the basic review ordinarily required of the corps.

The cost-benefit ratio, the standard but controversial measure used to determine if a project is economically sound, has been waived for some projects. Congress simply would declare them sound.

Standard cost-sharing arrangements with state and local governments on flood-control and dredging projects have been modified in some cases. The House bill would set another precedent by permitting 100 percent federal function of water treatment and supply facilities in Ohio and Oklahoma.

Private interests also are taken care of in the House bill. For example, it would authorize a $45 million scheme to deepen and widen nine miles of Gulfport Harbor on the Gulf of Mexico in Mississippi, mostly for the benefit of E. I. duPont de Nemours & Co., which is building a plant there.

With this largess touching every state and many congressional districts - in an election year, when goodies for the home folks count for something - little criticism is heard on Capitol Hill.

Of the 44-members of the House Public Works Committee, which handled the authorization bill, only two felt strongly enough to speak against it at the time of final passage - David E. Bonior (D-Mich.) and Robert W. Edgar (D-Pa.)

Bonior noted that 45 percent of the $1.25 billion in new projects was not approved by the chief of engineers, 13 percent modified cost-benefit ratios, 12 percent altered local cost-sharing provisions and 3 percent provided full federal funding of water systems.

"Everything the president has opposed - inflationary projects, expansion of government, environmental damage - is here," said Blackwelder. "He has plenty of reason for vetoing this legislation."