Ever since Olafur Eliasson debuted his Green River project (in which he introduced a nontoxic substance into a stream so that the water temporarily glowed a bright-neon green) in Berlin in 1998, rivers have been an obsession for him. Now, on a typically grand scale—I wonder whether anyone has ever challenged Eliasson to produce a work on a small scale, and whether he would be able to manage?—the Danish-Icelandic artist has created an entire riverbed in a wing of the Louisiana Museum. You walk down a long hallway with sterile white walls along a wooden floor, a sort of plinth, before