European rulers and engineers of the 19th century thought about it. The concept had been pondered even in Charlemagne's time, some 1,200 years ago. By the mid-1980s it should be a reality -- a navigable and commercial transcontinental river-and-canal network linking Northern and Western Europe with Eastern Europe.

The waterway will be some 2,400 miles long, a unique link between East and West Europe. It will connnect the Rhine and Main Rivers with Europe's longest natural waterway of all, the Danube, which flows east from Germany for 1,700 miles.

It will allow an uninterrupted flow of waterborne cargo between Europe's largest port, Rotterdam, on the North Sea,' and Black Sea ports, passing en route through eight countries.

This fast-developing East-West concept does not stop there.

* About 1985 the French are due to complete a Rhone-Rhine canal, complementary to the Rhine-Main-Danube system. This will create a direct link between the industrial heart of Europe and Marseille on the mediterranean and provide a connection to the longer canal system and to Eastern ports.

* The Russians have ambitious plans for 150 miles of canals linking up a string of lakes between the lower Danube and their Dnieper river. The latter is already connected through canals to the western Dvina and the Neman River, both of which flow to the Baltic.

* Longer term, the Yugoslavs and Greeks -- held back only by the cost of a formidable construction project -- plan a navigable waterway between the Danube and the Aegean.

Canals would join two rivers rising in the mountains of Macedonia -- the Vardar, flowing south, and the Morava, which runs north to join the Danube near Belgrade -- and thus open up a route from Central and even northern Europe to the Greek Aegean port of Thessaloniki.

* Three Eastern states are involved in more immediate projects. Yugoslavia, Romania, and Bulgaria are each hotfoot for modernization.

Yugoslavia, independent and with easier access to Western technology, is already on the threshold as a medium industrial state. Its two Soviet-bloc neighbors are pushing toward it.

All three -- often in cooperation -- are busily developing their middle and lower stretches of the river, not only for trade but also as a source of energy.

Romania has set a top priority through the 1980s to double both its Danube cargo and hydroelectric power capacities.

With Yugoslavia, it has already built Europe's largest hydro plant, Djerdap I , in the Iron Gates section, for half-shares in 10 billion kwh of electricity annually.

Downstream, the two are halfway through a joint project. Besides giving each of them another big power boost, Djerdap II is being equipped with the same high-capacity electronic locks as the first. They will speed transit through this once impassable section.

Even farther down, Romania is building its third Danube plant, this time with Bulgaria, with power stations on both banks (at Turnu Magurele and Nikopol) and a capacity of more than 4 billion kwh.

(Yet another "collar" is being put on the river by Czechoslovakia and Hungary , between Bratislava and Budapest, with similar energy potential for each.)

In Romania, with its immense Black Sea delta, the Danube lends itself to development more than elsewhere. Thus, Romania is pressing ahead with three new Danube rail and road bridges, a series of canals, including a direct Danube-Black Sea link (a Romanian construction "Gulag" claimed many lives before its abandonment in 1955) between Cernavoda and Agigea, and four major harbors.

One, at Agigea, will match Romania's biggest Black Sea port, Constanta.

Inevitably a decade of Danubian economic development and the prospect of still more rapid growth ahead is assuming interesting political overtones.

Danubian federation may be as dead as the dodo. But Austria as well as communist Hungary and Romania have all lately espoused the idea of wider, more general cooperation among the states the river passes through, not only for common economic benefit but also as a factor in regional and European stability and security.

Unlikely as it seemed just after World War II, when the Soviet Union was able to gain control of the river through its new communist allies, under pressure of economic necessity the Danube finally became an "open" river again.

Nonetheless, from Austria and Yugoslavia on, it still flows through a strictly controlled Soviet domain. The essential political-ideological conflict is unabated.

The constant passage of powerful tugs hauling their long lines of barges under the bloc's red flag -- already as far as Regensburg and Passau in West Germany and soon on to the north -- are a daily reminder of Russian's big economic interest. Even if detente recovers from its present coolness, one does not see the Soviets allowing East-West cooperation along the Danube going further.

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