The marshland (marisma), situated at the mouth of the Tinto and Odiel rivers, has been declared a World Biosphere Reserve. The habitat of swamps and sand, where the water level varies according to the tide (with a range that sometimes reaches 3m), provides shelter for a host of birds in spite of the industrial chemical facilities nearby. The site can be visited by canoe and small boat, taking in both sections: the Burro Reserve and Enmedio Island.