PRESIDENT CARTER'S attack on those pork-barrel projects has already had one result. We have found out how most of the Senate is going to be spending its time. "Nobody is going to be doing anything but trying to restore their water projects," Sen. J. Bennett Johnston (D-La.) declared last Thursday. Sen. Warren G. Magnuson (D-Wash.) agreed: "In the long run we will win . . . But its stops everything."

Having made their priorities clear, the senators whipped through an amendment to the public works bill telling Mr. Carter not to try to hold up spending for any water projects this year. The move was gratuitous, since Mr. Carter has not impounded any funds, but the message was blunt. Sen. Gaylord Nelson (D-Wis.), who has iconoclastic views about public works, did object to the hasty move, and he and 23 others voted "no." The prevailing spirit, though, was summed up by Sen. Daniel P. Moynihan (D-N.Y.), who announced he would be glad to support some dredging in Louisiana bayous in exchange for help on a public-works formula favorable to New York.

Meanwhile, Sen. Floyed Haskell (D-Colo.) has been stalling the confirmation of Guy Martin, a capable Alaskan, as an assistant secretary of Interior. Mr. Martin, when confirmed, will be reviewing reclamation projects, including several in Colorado. Sen. Haskell wants Mr. Martin to make up his mind about those projects in advance.

All this maneuvering is partly a product of pique. As majority leader Robert C. Byrd and others have tried to tell Mr. Carter, senators tend to be disturbed when projects of importance to their states are suddenly scuttled by officials downtown.

Yet Mr. Carter is well within his rights in removing those 19 projects from next year's budget and reassessing plans which, in some cases, are more than a decade old. The Interior Department has already concluded that for 27 of 45 reclamation projects, future costs -- if calculated at current interest rates -- would be larger than future benefits. Such problems are worth thinking about before more millions and billions are spent.