It’s increasingly difficult to make the case for the Thames as a cultural asset. Over the past century it has been an industrial thoroughfare, a back-end, an unlovely and often inaccessible strip of commercial offices, a conduit for inflating the values of apartments aimed at the super-rich and the backdrop to some of the worst, most ill-considered and incoherent architecture in the centre of any serious city anywhere in the world.

And yet. The “dirty old river” that The Kinks sang about in “Waterloo Sunset” keeps flowing and still, somehow, manages to spark surprising vistas, supplies a bracing breeze and cuts a slice of welcome space through the congested city. It is London’s most enduring thoroughfare and its most underused public space, a waterway that belongs to us all and yet seems somehow out of bounds. Canaletto painted it as a fanciful blend of Grand Canal and lagoon, a processional piazza. Whatever it is now, it is not that.

Its status as London’s last empty artery has provoked a procession of dim, doomed, one-liner plans, from the Garden Bridge (abandoned, after a spending of £53m) to the “dangleway”, an unneeded Docklands cable car (for both of which Boris Johnson was largely responsible when he was mayor of London).

One intervention, however, has been designed to bring a little finesse to the Thames. The Illuminated River is a project to light London’s bridges in a more coherent and more intelligent way. Designed by US artist Leo Villareal, it conceives the bridges as a kind of continuous artwork, a subtly changing light installation cutting through the city and tying the north and south banks together. So far, four bridges have been illuminated: London, Cannon Street, Southwark and the Millennium footbridge. Next month, Blackfriars road bridge, Waterloo Bridge, the Golden Jubilee footbridge, Westminster Bridge and Lambeth Bridge will be switched on too.

Villareal, who won the commission with architects Lifschutz Davidson Sandilands, in an open competition, gained global attention for his theatrical illumination of the Bay Bridge in San Francisco, which has become one of the city’s most popular sights. San Francisco, however, with its broad bay and epic vistas, is a very different proposition to the Thames, with its utilitarian Victorian railway bridges and surrounded by a jumble of medieval alleys, refuse depots, and waterfronts that are often privately owned and sometimes inaccessible. I walked from the Millennium footbridge where the illumination is a delight. The white LEDs pick up on the reflections in the ripples of the water below, an aqueous, gentle flow which echoes both the footsteps of people on the bridge and the gentle swell of the river, finally realising architects Foster + Partners’ original intent to create a “blade of light”.

The other bridges have been soaked in colour. Sometimes it works; the illumination of London Bridge emphasises its slender profile, highlighting an under-appreciated modern structure. It’s less successful on the chunky flanks of Cannon Street railway bridge, where the illumination of the riveted iron panels only emphasises the clumsiness of the Victorian engineering and their surprising disregard for the Thames. At Southwark Bridge, Villareal has done something more sophisticated in attempting to illuminate the shadowed nest of bracing and girders beneath the bridge, making visible its engineering and changing its character by night so that the structure appears delicate and complex.

The lights on Waterloo Bridge were being tested when I visited, an effort to illuminate another under-appreciated bridge, designed by the architect of Bankside Power Station (now Tate Modern), Sir Giles Gilbert Scott, and built during the second world war, largely by women. The fine white line I saw looked promising, not trying too hard to undermine one of London’s key vantage points.

The colours in light are, apparently, inspired by artists who have portrayed the Thames, from Turner to Monet, an impressionistic array of lilac and pastel pink, teal and gold — though there seems to be a misunderstanding here as these were colours cast by the sun and they look a little garish at night. They change gradually so that you need a few minutes to pick them up; there are no flashing fairy-lights or lasers.

Sarah Gaventa, the director of the Illuminated River project who took me on a riverside walk, told me: “We had to try to be very unlike the Garden Bridge. So this was not sensational and we had no public funding.” She later clarifies that there was a little public funding at the beginning: £500,000 from the City of London and £250,000 from the mayor’s office for seed funding. But the scheme, costing £31m in total, was otherwise entirely sponsored by philanthropy, kickstarted by Hannah Rothschild; those costs include maintenance for a decade.

“It’s an artwork which will be seen by 90m people in a [normal] year, completely free of charge.” Gaventa adds. It has also, she tells me, been a saga of almost unimaginable complexity, requiring collaborations between bodies, councils, transport companies and networks, the Port of London Authority and landowners, all of which underscores the odd status of the Thames as a kind of orphan element in the cityscape.

As public art it is, you might argue, making the otherwise banal infrastructure into a medium. But in its transformation of the everyday into art, it collides, awkwardly, with a city increasingly rebranding itself as an endless spectacle. The cascade of stupidly shaped skyscrapers, sky-gardens, Ferris wheels, domes, pop-ups and viewing decks seems to betray an insecurity, a sense that the city is not enough. It’s a phenomenon you see in Hong Kong, Shanghai and Dubai, in skylines reinventing themselves as entertainment; in the inevitable wake of falling demand for office space in the city post Brexit and Covid, it is a trend likely to accelerate.

The Illuminated River project has injected a glint into a riverside walk and, of the people I stopped to ask about it, was clearly popular and invigorating (even if only few understood that it was an artwork or that the colours changed). But it illuminates a city which seems to be losing its way, with no clear plan, with random clumps of towers appearing as if by accident, with a skyline being rapidly privatised and public space being outsourced to the private sector.

At the outset of the competition, the Thames was described in documents as “a ribbon of darkness”, as if something that was not transformed into an attraction had no significance or reality. The irony is that in highlighting the particularities of the bridges, their character and aspect, the essence of the city is being transmuted into something to be consumed; in its efforts to become more special, it becomes more universal. As Guy Debord wrote in The Society of the Spectacle (1967): “The spectator does not feel at home anywhere because the spectacle is everywhere.”