PRESIDENT CARTER'S effort to overhaul federal water policies was intended in part to restrict federally subsidized projects and curb consumption where water is scarce. But pressures from the states and Congress have been so strong, and the administration has backed off so far, that the outcome may be about the opposite: a vast new waterworks-building program without a real effort to raise users' charges or cut demand.

The clearest harbinger of this is the recent accord among western and eastern governors. For over a year, the westerners have been battling and bargaining to save specific water projects and ward off federal policy changes that might impinge on state and private authority over water management and use. The eastern governors recently jumped in largely because they have water problems, too. As staff writer Margot Hornblower reported this week, water systems in many eastern cities are decaying; replacing Boston's pipes alone could cost $300 to $500 million. Ground water in many areas is polluted or overdrawn. Strains on supplies have generated more and more elaborate plans for reservoirs and pipelines. New York City's $6-billion designs on the Hudson make many western projects look small.

There is the stuff of a classic coalition here. The governors have agreed on a national goal of water conservation - and their concept of conservation is elastic enough to stretch from stopping leaks in Boston to building dams in Utah to catch seasonal flows. We can easily see where this leads to a fat omnibus bill titled, let's say, The National Dam-Building, Pipe-Patching and Water-Saving Act. And we can envision the price tag, the congressional backing such a package might command and the administration's problems in trying to hold a budget line.

Opposing all these projects as "porkbarrel" - or "rainbarrel," if you wish - is, in our view, the wrong approach. Sone of the proposals may be very sound. But every dam and pipeline that is built from now on should be justified at today's prices and interest rates, with the beneficiaries paying a reasonable share. Beyond that, conservation-by-construction should be tied to serious efforts to protect ground water and to curb demand through water metering, irrigation efficiency standards and the like.

That is the kind of program President Carter set out to shape. The governors have resisted it under the banner of states' rights - and Vice President Walter Mondale and Interior Secretary Cecil Andrus have backed off a long way for the sake of political peace. The administration showed its accommodating spirit even more plainly last month by approving Denver's huge Foothills water-supply project without immediate efforts to reduce water use in that booming metropolis.

This is an easy path to trouble - budget troubles in the multibillion-dollar range, and the vexing prospect that another generation of big dams and water-supply systems may be built under the same old wasteful rules. Mr. Carter should take a much tougher line - and take it now, before the forces that seem to be amassing gain even more momentum and become even harder to turn around.