For his latest work, the Tate Thames Dig, he and his volunteer beachcombing team have grubbed through the river's slime and ooze at low tide for things to exhibit in one of his trademark curiosity cabinets. The two dig sites are Millbank, opposite the Tate Gallery which stands on the site of a huge 19th-century prison from which convicts were transported to Australia, and Bankside, which housed theatres, brothels and bear and bull-baiting pits in Shakespeare's time, and where the Tate's extension is nearly finished.
When I visited the dig the mud was like quicksand. Thick drops of rain fell and it was a sweat hauling buckets across the bridge to the Tate's storage hut. We could have talked in the gallery restaurant but Dion chose to sit on a plank outside the hut.
In his boiler suit, he looks like a cross between Woody Allen and Bill Gates. So far he has found an array of things including a plastic fridge magnet, animal claws and an undeveloped roll of film. He explains the inspiration for the dig. "It was the feeling that the river isn't really integrated in London. At certain points - I've seen it in New Bedford, the city I come from, and I've seen it here in cities like Liverpool - the city turns its back on the waterfront and moves in another direction and that's always seemed to me tragic.
"But the Metropolitan Museum in New York is integrated into its site in Central Park successfully and at the Tate at Bankside there's an opportunity for integrating the river. The Thames is a natural system which the city is entirely in debt to."
He values flotsam and jetsam as "links to concepts, philosophy made concrete. They guide you through the past, through ideas, through imagining other systems of value. Even the smallest item is a keyhole to another time". Dion intends grouping the finds of the Thames Dig visually - in terms of size and shape, for example - rather than chronologically.
He rejects a linear model of history. "I want to show the continuity of history instead of seeing our moment in time and history leading up to us. This is just one stop on a bus that's gonna continue far beyond us and I'm tracing these artifacts which are a shadow of how the city's grown and developed.
The river of course physically mixes history so Dion's approach is apt. His eye has been sharpened by decades of beachcombing. He can hardly walk down a street without discovering something and he rarely stops hunting and travelling. In the back of his mind, he is already plotting a future exhibition at the Carnegie Museum of Art in Pittsburgh. For his best-known work, A Yard of Jungle (1992), displayed in the Rio Museum of Modern Art, he went to the Amazon. And he made erhaps his most remarkable find, a pair of Renaissance shoes, turned up during a dig at the 1997 Venice Biennale.
His jetsetting seems glamorous but Dion shrugs. He obviously misses his wife, also an artist back home in their "Unabomber-style" Pennsylvania shack. He finds it hard to pin down the values which guide and sustain him from day to day. He says: "I am overwhelmingly a pessimist who wants to be proven wrong." About what? "About pessimism." Relating to? "Relating to the general state of the world."
This bleak view may derive from his career-soldier father who was involved in Second World War Pacific Rim combat, Korea and Vietnam. "I think a lot of Catch-22 kind of things happened to him." You can imagine. "He was part of a sniper pair. Someone spots the target and somebody shoots. My dad did the shooting. At one point, his partner went behind the rock one way and he went the other way ... and his partner didn't come back." He was blown up, Dion adds then goes quiet.
But at the mention of art again his spirit immediately lifts. He really likes contemporary art and a recent Mori poll shows he is far from alone: galleries and museums are now more popular with the public than concerts, theatre, opera, stately homes and theme parks.
Dion suggests "that's to do with the cult of the original. As society becomes more and more mediated and people are spending more and more time with representations, people do want to see The Thing itself. Even if a picture is a kind of representation, it's still The Thing - it's not a computer-generated picture in a website. There is this desire to see the original. Museums create this force around things and a desire for people to engage with that. And art objects don't pander - you engage them and unravel them on your own terms and I think people don't feel challenged by what they see on television, what they see on movie screens."
In creating his work, Dion is drawing on a wealth of artistic influences. "I could almost go from A to Z and give you someone for every letter." He actually begins to do this, starting with A for Aitken, the American videomaker. Dion is often associated with B for Beuys. Joseph Beuys too used chunks of real life in his art work, once making a cast of the corner of an underpass.
Both artists have made use of living creatures. Beuys exhibited himself with a coyote while Dion's The Library for the Birds of Antwerp (1993) comprised living birds surrounded by emblems of predation. But Dion dubs Beuys a curious kind of failure, in that, ultimately, his work always came back to himself and his presence was "so strong it overshadowed any artist following in his footprints".
The footslogging involved in the Thames dig does not trouble him - he is fit, hefting buckets with ease. The tough part is managing people and maintaining their morale. There are days when you find nothing. "There are days when you can really get skunked," says Dion. "Beachcombing is a lot like visiting a jungle: if you go there and you're looking for lions, elephants, tigers, you're going to be very disappointed. But if you refocus on butterflies and dragonflies, you'll be satisfied. "It's the same thing if you're here looking for chalices and gold rings - you're going to be sorely disappointed. But you can refocus onto willow pattern dishware and a beautiful bit of 19th-century design and the strange kinds of things that happen to screws and bolts when they get rusty. Then you'll be satisfied."
`Tate Thames Dig' discoveries will be displayed on the front lawn of the Tate Gallery, Millbank until 13 August. The final collection will be shown in the Art Now Room at the Tate Gallery, Millbank, in October