"All the rivers in the world together were not peopled and adorned with so many villas as this river." Pliny

You always knew it was there and, one day quite suddenly, it is, coursing through the days like an unknown theory of synchronicity or a new extrapolation of time.

Then it defies you to look at it and see only surfaces -- the waters, the traffic or the cloudy weather. These reflections surrender to the flood of centuries. It may be abandoned, stripped of its glory, but the Tiber is still the thread that binds Rome to its source.

Green as the first olive pressing, the Tiber is one of the great rivers of the world, a procession without end, usually stately but sometimes wild. Since 1875, when steep embankments of travertine slabs were built to contain its annual floods, it has become a parade without a crowd, qualifying it as one of the least-cherished waterways of any city in the world.

No boat ploughs its waters. Two barge restaurants and a handful of rowing clubs are the sum of amenities it offers to a city otherwise starved of social and sporting facilities. Thirty centuries of recorded history -- all but one in which the Tiber carried the Tuscany trade to the sea -- and all memories of its tantrums have faded.

Ask any Roman who are the great writers of the Tiber, and he may hazard Trilussa, the laureate of Trastevere. You examine his verse in vain. The rise and fall of empire, the blood feuds of towering dynasties, the assassination of kings, Papal iniquities and the violent lives of Roman watermen -- not a nursery rhyme commemorates them.

At midnight, it is the loneliest place in the world, a Nile without felucca, dragging the phantom centuries as it wades through the mist beneath the bridges. Between plane trees, dripping with moulted leaves, and the dark highway of the Tiber's dreaming, stone flags plunge down into the river's underworld.

There, the latest raw recruits from Scythia or Illyria, the tougher-skinned barbone and a few diffident prostitutes still keep their tryst with the Tiber's history.

Not surprisingly, its most attentive chronicler was a foreigner. The watercolours of Ettore Roesler Franz, a 19th century Italian artist who spent long periods in Rome, captured the life of the river as it was about to disappear for ever.

Rome had three ports then: the Ripetta and the Papal timber yards, which handled cargo from Tuscany and Umbria; Porto Leonino, near the Ponte Mazzini; and the Ripa Grande, near the bustling Sunday flea market at Porto Portese. Some 800 wooden coasters, breasting the Tiber's current by sail or buffalo power, berthed each year with manifests from Greece, France, Sardinia and Genoa.

Roesler's paintings fill in the gaps occupied by today's featureless walls. Fishermen, swimmers and helmsmen crowd the scene, while palace and tenement hem the river. Add malaria, and it must have been a poisonous place, but the water was highly regarded. Romans preferred the Tiber to the muck plumbed in over the Aurelian aqueducts. Pope Gregory XIII drank a glass every day, living to 84, while Donna Olimpia Pamphili, the sister-in-law of Pope Innocent X, swam daily near the Ripa Grande.

Tiber fish -- mainly bass, carp, shark and eel -- were once rated as highly as the wheat of Campania, the oil of Cassino and honey from Taranto. The river's outcast status means, at least, the resurgence of wildlife. Cormorants and herons shake the occasional eel in their bills, but it makes no difference to Nicolo, hauling out 2.5kg of carp beneath the Ponte Mazzini. He weighs it, nets it for half an hour and throws it back.

"The fear of the Tiber is a recent phenomenon," said Rosalba Giugni Laudiero, president of the environmental organisation, Marevivo, whose headquarters is a barge tethered beneath the Ponte Mazzini. "There used to be little beaches, stretches of sand and Romans had the habit of bathing. But the building of houses without sewage facilities and pollution from the River Aniene have put an end to all that."

The city, reduced to a muffled roar, slips away to reveal a secret oasis of sunlight, breeze and oar strokes. Rowers from the upstream clubs skim by, writing a chain of eights upon the water.

The Tiber's most powerful scourge is the winter flood. With all their engineering skills, the early Romans could not mollify their liquid god, though the Emperor Tiberius considered diverting the river to the west, bypassing Rome altogether. The plan was abandoned when citizens complained that it would "diminish the pride" of Father Tiber.

"The flood is always a moment of terror," said Ms Laudiero. "There are dykes to the north that control the river but, if the rain continues, the moorings are filled with fear. Last time, the water was higher than the arches of the Ponte Mazzini."

Nothing remains of the upstream Ripetta port, demolished to make way for the Ponte Cavour. But two pillars in the Piazza del Porto di Ripetta on the Tiber's left bank record every high water since 1495.

The worst occurred in December 1598, coinciding with a sirocco that prevented the swollen Tiber from draining into the sea. The waters rose to 20 metres, sweeping away most of Ponte Senatorio (now Ponte Rotto, the 'broken bridge'), along with 1,500 Romans. But it was the 1870 flood, stealing in while Romans celebrated Christmas, that launched the 25-year building programme which finally enclosed the Tiber in eight kilometres of travertine cladding and sealed its anarchy for ever.

"Our duties, essentially, are anything to do with the river," said Commandante Gerard Sepe of the Squadra Mobile's River Service. "Attempted suicides, successful suicides, investigation into motive, the examination of bodies. Was it a violent death? Or was he killed and thrown in the river?"

Sepe heads the Tiber's last surviving boat service, based behind the 10th Century monastery of San Bartolemeo on the Isola Tiberina - though you won't want to use it. A piebald crow, a carp and an Alsatian puppy guard the entrance to his domain. Stairs lead down past the slabs of marble that, in antiquity, decked the entire island in the shape of a galley.

In Horace's time, when there were only four bridges across the Tiber, the preferred spot for suicides was Ponte Fabriccio, which ties Isola Tiberina to the left bank. Nowadays, the action has moved upriver to Ponte Garibaldi, linking the districts of Torre Largo Argentina and Trastevere.

"On average, we get 30-35 attempts each year, Sepe said. "You see, many suicides are exhibitionists. On Ponte Garibaldi, it's certain someone will see you, run off and call the police. And if they do jump, there's still a good chance we can save them. The water's very deep under that bridge so they're unlikely to do themselves much damage."

But some get through. One unknown Roman, aged 50, washed up in early February on the Isola Sacra, a man-made necropolis situated where the Tiber joins the Tyrrenean Sea. Sepe estimated he'd been in the water for 12 days.

From the black iron of Ponte Palatino to the Ripa Grande at Ponte Testaccio, the traveller is on his own, with only the conspiracy of seagulls for company. Of Rome's largest port, the maritime Ripa Grande at Porto Portese, nothing is left but iron mooring rings and a deserted esplanade. The Papal jail, "where languished the victims of dark powers", is now a branch of the Italian Ministry of the Environment.

Across the Tiber, beneath the Aventine Hill, there was a lesser port, the Marmorata, where Carrara marble was landed to build ancient Rome. In 1867, two uncut blocks, lost during the unloading, were discovered on the riverbed. Now Danilo, a homeless man from Trastevere, supplements his budget by diving for marble chips, which he sells to collectors. He presented me with a coral-coloured piece, veined with white.

Seaward, toward the Ponte d'Industria, the banks are filling with shanty homes of box and cardboard while a rowboat clings to the shore like a wishful thought. The seagulls quarrel over sewage, gushing brown into the Tiber's foggy-green, forgotten waters.

Also by Michael Griffin:
Mission to cleanse?
Satellites and stars
Mr Sam and the food and beverage industry

Reaping the whirlwind: The Taliban movement in Afghanistan by Michael Griffin is published by Pluto Press (http://www.plutobooks.com/).