It may not be the grandest of the world’s waterways – the Nile and Amazon are ten times its length – but the Po has always exerted a fertile grip on the Italian imagination. Virgil called it ‘the king of rivers’; Dante died in its marsh estuary, having earlier described in Purgatorio how Jacopo del Cassero was chased there and fell fatally, ‘entangled in the mud and reeds of the Paduan swamp’, leaving a pool of blood on its waters.