Extension, location and municipalities:
With an extension of 957.29 ha, following the path of Pas river from its headwaters to the estuary of Mogro for more than 50 Km. Also includes its main affluent, Pisueña, as well as those of the headwaters. It affects the municipalities of Piélagos, Vega de Pas, Luena, San Pedro del Romeral, Santiurde de Toranzo, Castañeda, Selaya, Santa María de Cayón, Puente Viesgo, Corvera de Toranzo, Villacarriedo, Villafufre, Saro and Miengo.
Specific flow of 28 l/s/km2.
Pas river avoids an important slope in a short path, from the mountains to the coast. Its headwaters appear from a series of torrents in the occidental wall of the mass of Valnera, meeting in Vega de Pas, and then adjusting to the dominant ground lines.
This river goes west from its birthplace. It carries mountain waters from the passes of Escudo and Castro Valnera. It only receives left affluents during the first 15 Km, very sensible to rain quantities.
Once receiving Magdalena river and turning north, starts the development of a wide flow that reaches its maximum in Santiurde de Toranzo, while generating terraces of special importance between Villasevil and Iruz. Before meeting Pisueña river downwaters from Castañeda, it creates a small gorge in the Range of Dobra.