IMAGINE the River Liffey with no whiff at low tide, no used tyres, old prams, stolen bicycles and supermarket trolleys rising out of the slimy mud. Imagine the city stretch of the river, from Islandbridge to the Custom House Docks, as a sheet of water shimmering in the sunlight, with Amsterdam-style water buses chugging along beneath the bridges.

All of this would be possible if we were to build a barrage on the Liffey to celebrate the millennium. It would be a project of lasting value to Dublin, transforming the city's most memorable feature - the river immortalised by

James Joyce as Anna Livia Plurabelle. It would also make a lot more sense than building a useless civic monument.

At various times over the centuries, the idea of a barrage for the Liffey has been floated, without much success. J. W. deCourcy, in his wonderful book, The Liffey In Dublin, records that the monks of Kilmainham built a weir at Islandbridge shortly before 1220, to provide water for a mill-wheel. This weir, though not the mill-wheel, still exists today.

In the early 18th century, a barrage was proposed for Ringsend to facilitate navigation. By 1853, the stench of raw sewage had become the main justification for a barrage, with the Royal College of Physicians complaining that the deposits in the riverbed at low tide "taint the atmosphere to a great extent with the most offensive effluvia".

However, as Prof deCourcy notes, the idea of damming the river to get rid of the smell - which even the Viceroy supported - was dismissed by George Halpin, chief engineer of the Ballast Board. He pointed out that the Liffey is still the great main drain into which the sewerage of Dublin opens" and, therefore, a barrage would be impractical.

The discharge of sewage into the river ceased around the turn of the century, hut the unpleasant smell lingered on because of the volume of industrial effluent being carried into the Liffey by its tributaries, particularly the Camac. Since the closure of Clondalkin Paper Mills, however, the once-characteristic stench is no longer so evident.

IN 1975, when the Matt Talbot Bridge was being planned, a Dublin engineer, Conn Sheehan, proposed in Technology Ireland magazine that the design should incorporate a barrage to maintain a constant level of water upstream. The primary purpose was essentially aesthetic to create a pleasant public amenity for the city.

Mr Sheehan never believed that his proposal received a fair hearing from the relevant authorities. Dublin Corporation commissioned a report from the ESB, which concluded that it was not practical" to design a barrage which would retain water at a sufficiently high level without increasing the risk and severity of flooding in the city centre.

Though it recommended going ahead with the bridge, the report also said that an independent scheme for a barrage incorporating large flood-gates was "practicable", but it would need "considerable further investigation" and the remedial works required to deal with the discharge of storm drains upstream would be expensive.

Since last November, as a result of a chance conversation at the Royal Institute of the Architects of Ireland's annual conference in Berlin, James Horan - partner in Blackrock-based architects O'Dowd O'Herlihy Horan and head of the DIT School of Architecture in Bolton Street - has been working to flesh out a scheme which he believes should be "Dublin's gift to itself for the millennium".

HE has proposed constructing a weir to the east of Matt Talbot Bridge, topped by a pedestrian walkway which would link City Quay to the Custom House Docks site. It would also incorporate lock gates on its southern side to provide for water traffic, an artificial island adjoining the lock and a salmon run to allow fish to pass upstream.

James Horan sees the foot-bridge as a flat timber walkway which would sit on the weir structure, lit at night by a combination of overhead lamps and downlighters. "The weir itself would be an object of interest both sculpturally and also in terms of the flowing water noises generated by the cascade of fresh water into the tidal waters below," he says.

It would also create a natural pedestrian route from Pearse Station in Westland Row to the Custom House Docks. And with the river upstream held at a constant level, taking into account the clearances at the various bridges, a water bus service similar to those in Amsterdam could run between Heuston Station and the Financial Services Centre.

A new relationship between the Liffey and its quays would he made possible by the weir, with notable buildings such as the Four, Courts and the Custom House constantly reflected in a mirror like sheet of water. Indeed, Mr Horan believes it would even be feasible to "hang" a series of walk-ways from the quay walls just above the new water line.

Sean O'Laoire, the architect in charge of preparing a master plan for the redundant docklands area, was impressed by the scheme, which is more articulated than any previous proposal for a weir, or barrage, on the Liffey. "It's a fairly obvious thing to do because it reconciles a lot of things," he says. "It also offers a future for leisure boating on the river."

He notes that there are several precedents, such as the French spa town of Vichy, where a salmon weir doubles as a slalom run for canoeists. London has an elaborate barrage, which was designed as a flood harrier while Belfast recently acquired a colour weir which looks particularly dramatic when it is floodlit at night.

IN Glasgow, too, there is a well-developed plan for an "inhabited weir" on the Clyde, which would include a shipbuilding museum, bars and restaurants. Bidding for funds from Britain's Millennium Commission, the project is entitled "The River of Dreams" and aims to turn the Clyde - now viewed as a barrier - into an exciting "water park".

James Horan accepts that his proposal for the Liffey would have to be examined in more detail, probably through an environmental impact study. One of the principal risks would be that holding the city stretch of the river at a level equivalent to current high tide might raise the water table in the city centre, exposing cellars and basements to flooding.

"Even at the highest tide, there is still at least two metres between the water level and the quay wall and I can't imagine basements would be much lower than that," he says. The basement of the new apartment scheme straddling what's left of Isolde's Tower at Essex Quay is three metres below ground level high tide is a further 1.5 metres beneath this.

There would also be a need for relief valves to scour the riverbed behind the weir, but this technology is available. A related "downside" would he the need to deal with the outfalls of surface and storm water drains, as well as the Camac and Poddle, all of which might have to be intercepted before reaching the Liffey - a significant engineering job.

Given these considerations, it is not possible to put a price-tag on the weir at this stage. However, its aesthetic and amenity benefits to the city are so worthwhile that the Government should at least commission a serious study to quantify precisely what would need to be done to realise this grand project for the 21st century.

So far, the only Liffey-based project proposed to mark the millennium was the floating "Time in the Slime", which proved, to be a short-lived and embarrassing failure. By contrast, the project conceived by James Horan would be a solid and long-lasting legacy to Dublin. After all, the weir built by the monks of Kilmainham has lasted for almost 800 years.