Lagoons and Lagoons of Portugal
The third session of the cycle of conversations on geology, professor Galopim de Carvalho promoted by the National Museum of Natural History and SOL has been following this week about the lagoons and lagoons of Portugal. The opening, the warning that the exhibition would be more geographic than geological. For those who like to know what is what, which was used for both, when you go only by the names of the places, the accuracy is not guaranteed. There will be no shortage of examples, but let’s go to the first ones: our rias, for example the Ria Formosa in the Algarve or the Ria de Aveiro, are actually examples of lagoons, explained Galopim de Carvalho. Ria is the popular name, but the confusion arises because in geography rias were also coined, but they are not. The idea dates back to the 19th century, when the German geographer Ferdin and von Richthofen described how the Galician rias, detailed Galopim de Carvalho. «Some of us got an idea of how our rias would be geologically similar and they are not».
To understand why it is necessary to go to the concepts. «Lagunas, or paralic basins, are called few bodies of water, close to the coast, deep closed to the sea by a barrier». In Portugal, examples are the Ria de Aveiro, the Ria de Faro-Olhão (or Ria Formosa), the Ria de Alvor and the Barrinha de Esmoriz. In Galicia, where Ferdinand von Richthofen used to laugh, they spoke of something else fluently: the old vials of busy water lines. There, what happened was that over thousands of years, courses collapsed like water keys on a piano, creating a gap that the sea took advantage of. In the Portuguese ‘rias’, it was not the least that the sea climbed into the land, it was sand that was deposited in the formation of barriers, more or open, that created enclaves where the waters of the rivers meet the waters of the sea. «In Portugal, a barrier that closes the lagoons is usually sand, but there are other barriers, for a great barrier reef in Australia that closes an immense set of lagoons», showed Galopim de Carvalho.
Where did the idea of ria come from then? It was an old Portuguese word, before the definition of Richthofen, which means creek or great river. «These are the words that come from popular usage and that geography and geology ended up bringing to their world, to their lexicon. It is very interesting not to miss this history of the names that are part of the idea we are studying».
Visiting the Portuguese lagoons from a distance, 5 thousand years ago, as can be seen in the modeling below, the area where the Ria de Aveiro is today was a delta open to the sea and it is thought that it began to close in the 16th century, as a result of from the blowing of the wind and swell that was dragging sand and formed a barrier. A geological luck that dynamized the region, either for the salt pan, or for the capture of the moliço. «Today, tourism pays more to the moliceiro and is less tiring», joked the professor.
To the south, the Ria Formosa is unavoidable. It receives water from the Seco and Gilão rivers and the São Lourenço, Marim, Mosqueiros, Almargem and Cacela rivers and extends for 60 km from Ancão beach, in the west, to Praia da Manta Rota, in the east, extending – in the municipalities of Loulé, Faro, Olhão, Tavira and Vila Real de Santo António. A barrier is marked by two peninsulas (Ancão and Cacela) and five islands: Barreta, Culatra, Armona, Tavira and Cabanas. «These islands enclose marshes, which are sometimes covered with water, other times uncovered, being called marshlands and areas periodically flooded by brackish water».
Another thing you can take with you on your holiday to the Algarve this year is that this is one of the Ramsar sites in the country, a wetland classified as a place of international ecological importance under a UNESCO zone from 1971.
To the south is the Ria de Alvor, another lagoon that receives water from four streams born from the Sera de Mochique: Farelo, Torre, Odiáxere and Arão. Heading north, we arrive at the barrinha de Esmoriz or Lagoa de Paramos which, from a geological point of view, is still a lagoon. It receives the waters of the rivers Lambo and Buçaquinho – «rivers of minor importance that no one has ever mentioned, even us who at school and studied all the tributaries» – and also from the Ribeira de Silvade.
What is also a lagoon is the Lagoa de Óbidos, another example of how the old names remained: it is a lagoon because it has an opening to the sea, a lagoon is a lake of dimensions while closed, even if it can be artificially.
We have these too, of course. The Lagoa de Albufeira has a curiosity: before the Tagus flows into the Mar da Palha, the result of a system of parallel faults that tore in the direction it took, it was for everyone who arrived, recalled Galopim. But there is one fact that you can still mention: there are three lagoons in the Albufeira lagoon: the large, the small and the Estacada. The big one is more than 15 meters deep and is the deepest in Portugal. Being a lagoon, it is completely closed to the sea, but the dune cord is regularly opened in the spring. The Melides lagoon was once a day at sea, it would have been the fishing port in the 18th century, but today it is completely closed and is not open. A different case is the Lagoa de Santo André, which this year will not be open either, but which it usually is every year. In Figueira da Foz they are known as Lagoas de Quiaios, but the largest in the country is Lagoa de Fermentelos, another Ramsar site, located in the triangle of the counties of Águeda, Aveiro and Oliveira do Bairro.
What continues to wear the name of lagoon but has actually been a dam for 100 years is Lagoa Comprida, in Serra da Estrela. It was 1 intermediate in 1910 and construction began in 1912, with walls raised over time. «We civilized the lagoon and today it is an important water tank». Before, there was only one glacier time in the country: a natural lagoon, which takes us back by 18 thousand years, to the end of the glaciation in Serra da Serra. It was trapped when the glaciers melted and the water was left in the depths that the ice itself had dug up.
There are other ways for ponds to be born. The lakes that rain later in the Azores are formed, for example, in areas where volcanoes have hit the wood and after the rains – in Portugal they are visited mainly from the Azores and in mainland Madeira, volcanism can occur between Lisbon and Mama 70 million years ago, therefore, possible box, such as those that gave rise to the S. Miguel lagoons, for example).
But on the mainland another type of lagoons little known. They are the so-called poljes de Mira-Minde or the Nave do Barão, in Loulé – the geological terminology has remained with the Slavic name, which means field. These are regions that come and go: in this lagoon, the calculus massif as a lagoon explained Galopim de Carvalho. The rain seeps in, but when there is a lot of it, “it springs from the interior through the same holes through which it entered, like a soaked sponge where the water comes out on top”.
When asking questions, one was direct: what does the professor think of the dredging carried out in these systems, for example in Lagoa de Óbidos? Will they tend to disappear? “Dredging processes are always presented to nature. The sea and the river live in a certain balance, mobile, but if man dredges or builds, nature ends up being disturbed. It is intended to end up why the man wants, responding differently Galopim, that this is to have more consequences, and that, without a doubt, admit that a concrete answer. The rest time will tell.