FOR MORE THAN 35 years, one administration after another has tried to persuade Congress to impose user charges of some kind on the inland waterways network. This year, at long last, there appears to be a chance that Congress can be persuaded to do so. The opportunity ought to be seized. The subsidy that the federal government supplies to commerce on the rivers and lakes has gone on long enough.

The issue here is not just revenue for the government to make up for the money it spends to benefit particular groups and industries. The more important question has to do with more equitable treatment for competing industries. The barge industry is the only major mode of freight transportation that does not pay at least part of the bill for the maintenance of its right-of-way. The railroads have been paying all along, although they are now being aided by government loans and grants. And the trucking and airline industries both put up a substantial amount of money for highways and airports. The result of this difference, of course, helps to keep the cost of moving freight over water substantially lower than moving it over land or in the air.

The size of the subsidy is substantial. The Corps of Engineers builds and operates more than 250 locks and dams in addition to dredging channels to keep them deep enough for the bigger ships to use. The Coast Guard provides navigation aides and the Maritime Administration guarantees loans. Taken together, the federal subsidies to the waterways network approach a billion dollars a year.

Every effort in the past to reclaim some of this expense in the form of user charges - tolls on the locks or additional fuel taxes, for example - has been rejected because of the influence in Congress of key members whose home states profit enormously from river traffic. Indeed; the Senate Commerce Committee voted last month, 14 to 1, for a study of the whole issue after the Senate Public Works Committee had voted, 14 to 1, in favor of a user fee proposal advanced by Sen. Pete V. Domenici (R-N.M.).

Well, that's the difference between two very different constituencies - commerce and public works. In any case, it is hard to imagine a subject less in need of further "study." The question has been examined by the government at least 17 times since 1939 and congressional hearings have been held on various occasions since 1946. There is sufficient evidence on hand to demonstrate that user charges will not kill the barge industry, nor even cripple it substantially. The latest studies show that the imposition of such charges could cost that industry some 5 to 15 per cent of its business because its rates will have to go up. Rather than make a case against user charges, those figures suggest that the railroads have been right all along; the free ride the barge industry has received has helped it skim off freight that, on the basis of non-subsidized costs, would have gone to the railroads - and thereby would have reduced the railroad's need for government help. It is time for Congress to face that and to stop spending general revenues for the purpose of helping an industry that is now strong enough to help itself.