Bonn — Ever since the days of the Holy Roman Emperor Charlemagne, men have dreamed of linking the North Sea to the Black Sea with an inland waterway stretching 2, 125 miles across Europe.
Now, more than a millenium after his death, the project is only 22 miles from completion.
But the dream has turned into a nightmare for West German politicians. Studies show that if completed, the canal linking the Rhine and Danube Rivers will lose money, drain much-needed business away from the loss-making West German railways, and spoil one of the purest landscapes in Bavaria.
When Bonn and the Bavarian state government signed an agreement to build the canal in the 1960s, it looked like a bold step to open up a major East-West trade channel linking 13 countries. West German officials believed it would create thousands of spin-off jobs in an economically backward area of the country known to humorists as the ''Bavarian Congo.'' Bonn pledged to pay two-thirds of the costs, which are now estimated to reach almost 4 billion marks ($1.6 billion) if the waterway is ever completed.
But water freight has declined rapidly, due to improved road and rail links. Latest forecasts suggest the canal would carry less than one-sixth of the traffic originally envisaged. And West German ''Eurobarges'' might even suffer from the link, since East European states are pressing to have the canal declared an international waterway, freeing freight rates for cutthroat competition from their subsidized barges.
The Bonn government argues that since the canal is entirely on West German soil and joins two rivers with their source in West Germany - the Main and the Danube - it is a national waterway. If this claim were upheld, Bonn would at least be able to fix minimum freight rates. But north German seaports could still suffer when the canal links Austria, Hungary, Yugoslavia, Bulgaria, and Romania directly to the Rhine delta - and to the Dutch ''superport'' of Rotterdam.
The canal project has also become a major target for West Germany's strong environmentalist lobby. Ecologists argue the waterway would not only deface the scenic Altmuhl Valley near Nurnberg, but also endanger rare birds and fish. Experts say that once completed, the canal would lower the water table around the Danube and drain ecologically important marshlands. Candidates from the radical ecologist ''Greens'' Party won a big local vote in recent Bavarian elections after campaigning on these themes.
Earlier this year, with the Bonn government looking for ways to save money, Social Democratic Transport Minister Volker Hauff persuaded the Cabinet to freeze funds for the canal and seek agreement with Bavaria to put a stop to the project. Mr. Hauff called the canal ''the dumbest project since the Tower of Babel,'' a phrase that incurred the wrath of Bavaria's powerful right-wing premier, Franz Josef Strauss - one of the canal's staunchest backers.
But the new conservative Bonn government that took power in October is having second thoughts about winding up the canal project. The new transport minister, Werner Dollinger, is a Bavarian himself and a prominent member of Mr. Strauss' Christian Social Union (CSU). He has just received new studies which suggest that although the canal may never be profitable, it would probably cost more to scrap the project now.
Accordingly, the new government has just provided 105 million marks ($41 million) in the 1983 budget for the canal. But the funds will cover only the cost of keeping the project alive, paying salaries, and meeting maintenance costs. There is not a pfennig (cent) in the budget for building work.
The present solution satisfies nobody. The Bavarian government is still adamant that the canal must be completed, and Mr. Strauss has made clear he will make the project an issue at the general election next March, when he hopes to be voted into Chancellor Helmut Kohl's Cabinet.
West Germany's neighbors, particularly Austrian Chancellor Bruno Kreisky, are also pressing for the waterway to be concluded. The Greens, on the other hand, are determined to prevent completion, if necessary with a campaign of civil disobedience. Finishing the canal, at present estimates, would cost Bonn about 1 .7 billion marks ($700 million) over the next few years.
So there it sits, a monument to the fallibility of planners and the indecision of politicians - a lot wider but, for the moment, just as inert as the Main-Danube canal that Bavaria's King Ludwig the First opened in 1855.
That waterway, a narrow and complex system with 100 locks, was rapidly overtaken by the railways and closed to commercial traffic in 1945. Overgrown with weeds, it runs parallel to the modern canal and not far from the remains of the ''fossa carolina,'' which Charlemagne started building with an army of 6,000 laborers in the year 793.
Whether this third and most grandiose attempt to link the North and Black Seas also ends as a ruin seems likely to be decided sometime next year.