Long-standing plans to divert the waters of northbound rivers to the south, the most ambitious and controversial of the Soviet Union's irrigation projects, are in trouble, victims of new economic thinking.

A deputy chairman of the State Planning Committee, or Gosplan, told a press conference today that at least one of the river projects costing billions of dollars was no longer necessary, and he raised doubts about another.

This confirmed informal reports that the plans to shift water southward from rivers both in European Russia and in Siberia had been dropped from the 1986-1990 economic plan that is to be adopted at the current congress of the Soviet Communist Party.

"We can manage without the switchover of Siberian rivers," said Leonard Vid, the deputy chief at Gosplan. He indicated less definitively that the European project involving the Volga River had been shelved, too.

During the past year, the rivers projects became a recurring theme, invoked as symbols of carelessness and waste associated with old ways of "extensive" economic planning.

Now that Soviet leader Mikhail Gorbachev has shifted to "intensive" development, the days of mammoth "hero" projects seem to be over.

The turning point in the rivers debate occurred recently when a group of economists lambasted the projects as extravagant and wasteful. Among those signing that article in the official party newspaper Pravda was Abel Aganbegyan, a leading economist whose ideas are considered in tune with Gorbachev's.

But the rivers projects already were the focus of a campaign by Soviet writers and cultural figures to preserve cultural monuments and protect the environment.

The European project was accused of threatening churches and other historic buildings in the ancient Vologda region. According to one report, 368 of 492 historical and cultural monuments in the region would be destroyed or damaged by the river reversal project.

One writer, raising the issue last December, said that in 50 years, he had never seen such differences of opinion on a subject.

"The rivers in question have been flowing for thousands of years," said Sergei Zalygin. "Why are we hurrying to reverse them this very instant?" He noted that the debate had boiled down not to the "pros and cons" but "who said 'pro' and who said 'con.' "

On the other side of the argument are the ministries involved in the planning and the republics of central Asia and other southern regions that need new sources of water.

According to one report, the European project -- taking water from northern lakes to the Volga River and down to the Caspian Sea -- would cost "at least 100 billion rubles" ($130 billion at the official exchange rate) by the time it was completed. Vid said the purpose of the project, to raise the level of the Caspian, already was occurring naturally.

The Siberian project, involving the construction of a canal 1,500 miles long to take waters from the mighty Ob and Irtysh rivers into central Asia, was even more ambitious and even less definite. It had not yet even received final approval, although all recent official comments had indicated it was still under study.