THERE IS A conservationist in the White House now, and he is willing to take on the powerful interest and officials allied with the Bureau of Reclamation and the Corps of Engineers. That is how environmentalists read President Carter's decision to delete 19 big water and power projects from his budget this week. To those whose pet projects have been undercut, the message is even more blunt: There is a new President now, and the standards for dams and water diversion projects are going to be much tougher than before.

The 19 projects illustrate the kind of economic, environmental and safety problems that do deserve review. There is that tired perennial, the Dickey-Lincoln project in Maine. There is the Auburn Dam, upstream from Sacramento, which may be vulnerable to seismic shocks. There are the two huge projects that, among other things, have aroused the greatest international concern - the Garrison Diversion Project in North Dakota, which is a suject of dispute with Canada, and the Centreal Arizona Project, the $1.5-billion-pus granddaddy of them all. And there are others that are barely justifiable under lenient rules and probably cannot survive a fresh accounting of their costs and effects.

Environmentalists should not, however, be too jubilant too soon. As the administration has been telling outraged congressmen, the projects have just been suspended so far. Interior Secretary Cecil Andrus has promised fair hearings to everyone - including House Interior Committee Chairman Morris K. Udall and Minority Leader John J. Rhodes, who are not about to let the Central Arizona Project die without a fight. By April, when his final recommendations are due, Mr. Carter may find some projects more tolerable, for one reason or another, than they seem this week. Moreover, public works is one area where the maxim really applies: The President proposes but Congress disposes - and Congress may be disposed to go on damming no matter what Mr. Carter prefers.

Thus some projects are likely to get financed anyway. But that does not reduce the President's action to a public-relations ploy. From all indications, Mr. Carter is a serious conservationist, far more in tune with environmentalists' attitudes than any other recent chief executive. His advent has already tipped the balanced in the executive branch, so that the engineering bureaus have lost influence to their critics in the Office of Management and Budget and the Council on Environmental Quality.

It is much easier, though, to throw cold water on dams than to develop and sell conservation-oriented alternatives, especially for Western regions where the competition for limited water is so fierce. Environmentalists, more experienced at dissenting than at governing, now have their best change yet to reorient national policy. To do so, though, they will have to make some hard choices and win much broader public and congressional understanding and support. It will be a stern test of the movement's maturity. The fights over those 19 projects will be only the start.