The present valley of the Ebro was flooded and lay under a big lake for some time. When the Ebro tunnelled through the coastal Catalonian mountain range, the lake dried up and just a few marshes remained in the depressions, with no drainage and only watered by rainfall. The intense evaporation gave rise to a high increase of salt concentration. In this way the “lagunas” (small lakes) of Navaridas, the downfolds of Elciego and Oyón, and, in Laguardia, the “lagunas” of Carralogroño and Carravalseca appeared.
The impossibility of draining its waters outwards and its importance as a refuge for a very interesting Flora are the reasons why, at present, they are considered as a Protected Biotype.