KEEPING VENICE ALIVE. Floods still threaten this island city. But Venice faces an even bigger problem -- a dramatic population decline. As a city-museum, Venice faces a bleak future. Although various groups are studying flood-control plans and proposals to halt pollution, the task of making the city livable remains.

THE threat to Venice is not that it may disappear under the tides, as was once feared, but that it will cease to be a living city. As a city-museum, Venice has a somber future. To confront its problems -- some natural, most man-made -- Venice must remain a city where people live, work, and prosper. It must be a city that is fully alive.

Once La Serenissima, the ``Queen of the Adriatic,'' ruling the seas and controlling trade with the Orient, Venice has become a city of hotels, boardinghouses, public buildings, palaces for the wealthy, and store after store catering to the tourist.

``Venice's future? What kind of future is there when the people are leaving,'' mused Giuseppe Rosa Salva, head of the Venice branch of Italia Nostra, an organization dedicated to safeguarding the nation's cultural heritage.

Venice counted more than 170,000 inhabitants before World War II. Now there are just over 80,000, and the trend is downward. A third of the population is more than 60 years old; fewer than one-third are under 20.

``Of the 18 couples I married last year, 11 have left the city,'' says a priest at San Giovanni Church here.

Is there still time to reverse the declining population? The answer may be yes. But it almost seems as if Venice represents a typical case of too many cooks spoiling the broth.

The Italian government, the regional government, the provincial government, the municipal government, the superintendent of fine arts, and the magistrate of the waters all have their own, often conflicting, ideas of how to administer the city. The result, so far, has been much talk and little action.

The Italian government has apparently decided that things have gone far enough and the time has come to save Venice. A consortium was formed a couple of years ago, called ``Venezia Nuova,'' grouping 27 Italian public and private organizations, entrusted with planning a comprehensive plan to revitalize Venice by the end of the century. On paper, its project may be feasible, though the cost seems prohibitive: $2.5 billion.

But there have been so many such projects and plans in the past that many Venetians are skeptical.

In at least one case, past inaction was due not only to lack of coordination, but to a clash of interests. The problem was the decision by the prewar Fascist regime to incorporate the neighboring city of Mestre into the municipality of Venice.

Although the population of Venice has been falling, Mestre has been growing, from 45,000 in 1939 to 200,000 today. Elected officials cannot be insensitive to the fact that there are more than twice as many voters in Mestre than in Venice.

Nowadays, it seems, what is good for Mestre and the surrounding regions is bad for Venice.

In Marghera, which was once an agricultural region of Mestre, the Fascist leaders set up a number of industries (there are now about 200) in the hope of turning Venice into an industrial city. Government owned or controlled, most of these industries lose money. But they provide employment for 25,000 workers, and that is important to the left-wing administrations that have governed Venice for years.

In addition to employment, these industries cause pollution that is endangering some of Venice's most beautiful monuments; they make the city sink by drawing water from beneath it; and they increase the threat to the equilibrium of the lagoon that surrounds Venice.

The industries have helped change the equilibrium and the ecology of the lagoon surrounding Venice. And the life of Venice is inseparable from that of its lagoon.

For centuries, governing authorities have undertaken projects to preserve the harmony between the islands and the lagoon. That harmony no longer exists.

To allow for shipping of raw materials to the industries in Marghera, authorities dredged a channel for ships of large tonnage and the three mouths connecting the lagoon to the Adriatic Sea were widened. Another channel was dredged to serve the Port of Venice. The result of these operations was to increase considerably the volume of water entering the lagoon, causing its level to rise in general.

To complicate matters, the area of the lagoon has been reduced by enclosures for fish farming and reclamation for agriculture. Thus, the open water of the lagoon is only 70 percent as large as what it was at the beginning of the 19th century. THE combination of the increased volume of water entering the lagoon, the reduction of its size, and changing weather conditions has resulted in floods in Venice of unprecedented proportions. Piazza San Marco floods when the tide is more than 28 inches above normal. Between 1970 and 1979, that level was surpassed more than 1,000 times.

The city is built on piles driven into silt. Standing in the midst of a lagoon and exposed to tides and storms, Venice's very foundations are undermined by the floodwaters.

The difficulty of living in Venice when cellars and ground floors are flooded, and the economic and social life is disrupted, is plainly evident. Even without the threat of periodic flooding, living in Venice can be challenging. A large number of apartments retain extremely primitive conditions and lack the most essential conveniences.

Modernizing an apartment in Venice is far more expensive than in most other parts of Italy. Rents for even renovated apartments are controlled at very low levels. Projects, no matter how modest, require special permits that can take months or years to obtain. Faced with the difficulties of renovating, owners with empty apartments often prefer to leave them vacant. The city's vacancy rate is over 10 percent.

For Venice to thrive and prosper as a living city, a way must be found to control the lagoon's water level. The study of this problem is a task the Venezia Nuova consortium is supposed to undertake.

A number of solutions have been proposed. One has already received government approval. The project calls for fixed engineering works to reduce significantly the width and depth of the three mouths connecting the lagoon to the Adriatic. Mobile floodgates would then be built on the bed of each opening so that the mouths could be closed off in the event of a storm surge. The project is very costly and, judging from past experience, may be a long time coming. ENGINEERING experts agree that reducing the level of the lagoon will not be enough to save Venice. At present, the rapid movement of the tides from high to low has washed discharges out to sea. This is why the city has never had sewers. When the movement of tides is regulated, the construction of sewers will be necessary. Also, the discharge of industrial wastes from Marghera has caused a spread of algae in the lagoon that is altering its ecology. A solution for this must be found, as well.

Augusto Ghetti and Michel Batisse, two experts intimately acquainted with the problems of Venice, have put it this way:

``It is clear that Venice's magnificent buildings can be safeguarded only by the overall protection of the city and the preservation of its intellectual, commercial, industrial, and tourist activities. In turn, this protection depends on a modern, rational management of the whole lagoon, . . . which makes the best possible use of its many resources for the benefit of all.''

On Nov. 4, 1966, a tidal storm of exceptional height and duration caused the entire city to sink under more than three feet of water. A cry of alarm was heard around the globe. Was one of the world's most beautiful cities nearing its end?

Much has been done to restore monuments and works of art since then. Effective measures have also been taken to stabilize the level of the city itself and to reduce pollution. All this has helped preserve Venice physically. The preservation of the city as a living community, however, may still be a long way off.

Venice lives today on a single industry: tourism. It keeps the city going, but is not enough to secure its future. For Venice to thrive, not only must the floodwaters be tamed, but the population decline must be halted. The latter may prove to be the more difficult task.