Tanzania's capture of Kampala today effectively ended post-colonial black Africa's most publicized-and perhaps bloodiest-dictatorship.

But Idi Amin, if no longer the arrogant, confident, self-proclaimed president for life ane "conqueror of the British Empire," was still at large and capable of wreaking further havoc.

The most immediate fear was that Amin might blow up the Owen Falls dam at the source of the Nile River near Jinja, where he is reported to have taken refuge.

The dam provides all Uganda's electric power needs and 15 percent of neighboring Kenya's.

But even discounting such a Goetterdaemmerung performance, the legacy of Amin's eight years in absolute power have left Uganda's social and economic fabric in tatters, with many of its 12 million citizens hungry.

Nor is there any guarantee that Amin's successors will be able to put the country back on a sound political footing.

Waiting in the Tanzanian capital, Dar es Salaam, to fly to Kampala are the members of the recently formed Executive Committee of the Uganda National Liberation Front.

As the organization's name suggests, its unity extends only to ousting Amin, and its members are a potentially explosive combination of exiled politicans ranging from Marxists to erstwhile monarchists and from idealistic socialists to practical capitalists.

Even now that the exiles have transformed themselves into a provisional government, theirs is likely to be a weak enterprise.

Waiting in the wings is Milton Obote, Amin's embittered predecessor who has not forgiven them for shoving him aside.

Ever since his overthrow by Amin in 1971, Obote has enjoyed the fovor of Tanzanian President Julius Nyerere, who now has effectively ousted Amin and could conceivably push his old Ugandan ally forward.

So far Nyerere has studiously refrained from doing so despite both men's dedication to radical socialism.

Whether Nyerere likes it he is likely to be calling many of the shots in Uganda in coming months.

If past performance is any indication, Nyerere is a man who would like to keep a low profile. But with the Ugandan Army in tatters, disgraced and scarcely in much public favor after eight years of military-enforced terror, rebuilding a military establishment capable of maintaining law and order may prove a formidable and thankless task for Tanzania.

Furthermore, the more Tanzania is seen to extend its influence in Uganda the more it is likely to irritate neighboring Kenya.

Whatever Kenya's unhappiness with Amin, any regime in Nairobi will remember the Obote days when Uganda and Tanzania tended to gang up against Kenya in squabbles that finally killed the East African Community inherited from their British colonial masters.

Now that Tanzania has closed its borders with Kenya-and cut off traditional markets for Kenya goods there and in Zambia-Nairobi has come to count on Uganda, Rwanda, Burundi and eastern Zaire as commercial outlets. These could now be threatened by Nyerere's influence.

More immediately important for Uganda is the question of whether hundreds of thousands of exiles, many of them highly qualified, will return home and start reconstructing their country.

Ravaged roads and facilities, an entrepreneurial class depleted by Amin's ouster of 40,000 Asians in 1972, the slaving of as many as 300,000 Ugandans, and the risk of bloody settling of accounts are other factors threatening Uganda's immediate chances of stability.

The West, which has long clucked its tongue in disapproval of Idi Amin's bloody anties, now faces the challenge of helping Uganda get back on its feet.

In Nairobi, no serious observers are taking any bets.

"without Amin, Uganda risks being dropped from the headlines," one diplomat said, "and becoming just another tiresome . . . African country going nowhere."