When Floyd J. Fithian was elected to Congress in 1974, one of the first objectives was to get a long-planned Army Corps of Engineers dam completed back home in Indiana.

Fithian a former history professor at Purdue University, knew good politics when he saw it; a dam built today is often a legislative career saved tomorrow.

The $108 million flood-control project, near the city of the Lafayette, on a tributary, of the Wabash River, was authorized by Congress in 1965, Nine years later, when Democrat Fithian got to Washington, planning for it had cost taxpayers about $1 million. But in 1976 Indiana Gov. Otis Bowen and the state legislature threw a curve. Largely on economic grounds, they came out against the reservoir and stopped funding the state's share.

Remembering his politics, Fifthian got religion quickly. He, too, came out against the dam and set to work to get Congress to "deauthorize" it -- that is, remove it from the corps' list of active projects eligible for funding and completion.

Once deauthorized by an act of Congress, the Lafayette dam would be dead. And the local government then could go ahead with its own plan to develop a recreation program around the site.

But nothing in Prof. Fithian's textbooks prepared him for the Washington reality he would encounter as Rep. Fithian.

Once Congrss authorized a federal water project, getting it stopped becomes a superhuman act. Corps project lists are larded with building schemes that go back decades and have no chance of being completed -- but which Congress will not deauthorize.

"It is part of the concrete mentality around here," said Peter Carlson, a water lobbyist for the Environmental Policy Center. "Deauthorizations make congressional committees and the Corps of Engineers look bad. They seem unwilling to recognize that conditions and needs change."

Fithian learned this by experience. He introduced a bill to deauthorize the Lafayette lake. It got nowhere at the House Public Works Committee, which sits in judgment on all such actions.

Last year, Fithian offered his proposal as an amendment to a committee bill on the House floor. It was not accepted by Rep. Ray Roberts (D-Tex.) chairman of the water resources subcommittee.

Then alond came Rep. Bob Michel (R-Ill.) this year with a different approach: a bill that would deauthorize 16 corps projects that had lingered for years -- including a $600 million plan, covering part of his district -- to build duplicate locks on the Illinois River.

A Michel aide said: "Bob was able to stop funding of the locks program in 1976 when the corps wanted to continue. He felt it wasn't necessary; it was outdated, and it would have adverse effects on the shoreline."

But the only way to stop the project completely and avoid yearly hassles at the Appropriations Committee was to get it deauthorized. Hence, Michel's bill, which he broadened to include other dubious projects.

One of those on his private "hit list" was Lafayette Lake, in Fithian's district. Fithian eagerly supported including Lafayette in the list of projects to be scuttled.

But when Rep. John T. Myers (R-Ind.) heard that Michel had put Lafayette on his deauthorization list, he went through the roof, sources reported. Myers, the ranking GOP member of the important water appropriations subcommittee, warned Michel to leave Lafayette alone.

Michel removed Lafayette from his bill, which, even without the Indiana project, continues to languish at Robert's public works subcommittee for water resources.

"John opposed deauthorization of Lafayette because it would provide some important flood-control benefits in our district," a Myers assistant explained. "We feel it will have to be built eventually . . . All since about $1.2 million has been spent on it so far, that money would be lost with deauthorization."

Added the aide: "John also felt that there is a well-established procedure for deauthorizing projects -- those that have received no money for eight years are reviewed by the Public Works Committee and deauthorized if no on objects."

That procedure, established by Congress in 1974, has resulted in the removal of dozens of ancient corps projects from the authorized list. But dozens more remain authorized because of individual legislator's insistence that they be left alone.

The Roberts subcommittee's 1979 water projects bill, expected to reach the House floor soon, would deauthorize four projects including the controversial Dickey-Lincoln hydroelectric dam in Maine.

But Lafayette Lake or the Illinois River locks program are not among the committee's select four. Lafayette, at least, is expected to have a better chance of deauthorization in the Senate, where both Indiana senators agree with Fithian's stand.

But then again, Lafayette Lake may be like any one of the other antiques that continue on the authorized list primarily because some member of Congress can't bear the thought of turning his back on a water project for a home district.