Although its geographical description does not mention in deep its biological importance, it talks of it as a physic frontier between the regions of Asturias and Galicia. This river is placed between the councils of Castropol and Vegadeo, in the western Asturian limit and the one of Ribadeo in the eastern Galician limit. According to its geographical description this river is the final part of a biggest river that is born in Galicia with the following coordinates, 43º 28′ and 43º 33′ of North altitude and 7º 00′ and 7º 03′ of western longitude. The river Eo is born in Fonteo and ends in the mentioned river before after 87 kilometres of route. The final estuary forms a wide water area with 930 square kilometres of extension, 800 metres of width and almost 6 miles of longitude.
This territory is rich and polysemous. The shape of this river is made thanks to a longitudinal fault and a series of minor fissures that form coves. Its current configuration comes from a series of processes of excavations and fillings of the valley. This process shows marine sediments, winds, tides and fluvial erosion. Its floor is made of slate, sandstorm and quartzite and, also in the surroundings we can find limestones and dolomites.
History tells us that the banks of the river Eo were the place for some civilizations, towns and nomads. The Romans, Normands, whalers and English pirates sailed on this water and stepped its sand. The last ones in 1719 were chasing two war Spanish frigates and, finally sank them under the water of this river underneath the Tower of La Atalaya, sacking the towns afterwards.
Sociologically and economically speaking this river is an important point. It generates moods of production, is also an important tourist point and at the same time, it is a vivid stage of marine customs and traditions.
It is also important to mention the importance of its ecosystem. This is clean and vivid and is the source of life for many species. It is an important area for 50 different bird species which come from the North of Europe to spend here the winter. Among these species we find the king duck and other animals of this kind. But many other species also spend here the whole year: herons, plovers, cormorants Occasionally we can find spoonbills and other rare species. This ecosystem has national importance. It was declared Partial Natural Reserve and Special Protected Area for birds, (Zona de Especial Protección para las aves ZEPA).
Its nature covers much more than birds. Here we can also find salmons that come to this river at the end of winter and stay here up to the beginnings of summer. Here there are also trouts. Among mammalians we may mention the otter. And in the high basin of it we can find birds of prey, roe deers, wild cats and genets.
Among its vegetation we can admire the wide submarine grasslands. In its interior there are extensions of rushes and reedbeds.
On its whole this area offers many attractions such as rocky places, beaches and coves.
But in addition to all this, this river also offers thousands of adventures for sports lovers. As it is a navigable river we can practise here fishing sports with a boat with oars or sailing. In the small ports of Castropol, Figueras and Ribadeo we can rent small boats to sail.
In Castropol there are the Sail and Sea clubs which organize competitions such as the popular regattas of traineras «Trofeo Bandera Prince of Asturias».
Another event for spare time is the excursions by nature. As the road surrounds all this protected area, we can admire this river on foot, by bike or by car making a round movement of 25 kilometres. A few kilometres from Ribadeo, by the national road N-634, the observatory of Reme is located. It is an excellent place to enjoy the landscape and the birds which inhabit this river, and we can see a big sign where all the names of species appear. From Castropol we can get to the cove of A Berbesa where there are clams and seafood which is usually caught by selfish gatherers. But this place is also a wonderful viewpoint to observe the unique panorama of this coast.