It is the area that lies between two of Galicia’s most significant water courses, both interms of their volume of water and of the wealth of fauna, flora, landscape and culture they hold: the Ulla-Deza River System, which marks the southern boundary, and the River Tambre and its tributaries, which form its northern limit.

The Ulla river has a basin measuring 2,764 sq. km, the second largest in Galicia. It has a total length of 132 km and a wealth of fish that includes species such as common Atlantic or European trout (Salmo trutta), Atlantic salmon (Salmo salar), lamprey (Petromyzon marinus) and brown trout (Salmo trutta trutta). Although significant in number but nevertheless present in these waters are species such as saboga (Alosa falax), shad (Alosa alosa) and boga (Chandrostoma duriense). Its main tributaries are the rivers Deza (51 km), Toxa (17 km), Liñares (15 km), Furelos (26 km) and Sar (39 km).