Where the Guadalquivir finally reaches the Atlantic, the river loses its way, spreading into marshes, dunes and pine woods that make up Doñana, one of the great wetlands of Europe. The park is a refuge for creatures you do not expect to see in southern Spain: the Iberian lynx, the world's most endangered cat, the Spanish imperial eagle, and in winter, hundreds of thousands of migrating geese and ducks. Flamingos feed in the shallows, and wild boar and red deer move through the cork oak woods.
Visiting means going slowly. Most of the park is closed to private vehicles, and the best way to see it is with a guided tour in an off-road vehicle, which you book in advance. The marsh itself is flat, wide and bright, with long skies and the smell of salt and wet earth. Nearby is El Rocío, a village of sandy streets with hitching rails, which at Pentecost welcomes a vast pilgrimage of horse-drawn carts and riders.
The riding on the way is mostly flat and straight, through pine forest, farmland and the great strawberry fields that cover much of this region. It is not demanding, but the heat is, and so is the wind that comes off the sea. Late spring and autumn are best, when the marsh is lively with birds and the temperatures are bearable. Midsummer can be fierce, with the horizon shimmering.
The closest sizeable town is Sanlúcar de Barrameda, with about sixty-five thousand people, at the river mouth, with fuel, seafood and the pale, dry manzanilla wine that is aged in the sea air. Fill up there, take water and mosquito repellent, and be prepared for sand on some of the minor roads. At dusk, the light on the marsh is magical, and the chorus of frogs and birds is something to remember.