The region has a humid oceanic climate, with warm summers and mild winters.Annual precipitation is around 1,200 mm at the coasts and higher in the mountains. Snow is frequent in higher zones of Cantabria between the months of October and March.
Towards the south are higher mountains, the tops of which form the watershed between the drainage basins of the Rivers Ebro, Duero and the rivers that flow into the Bay of Biscay.
These peaks generally exceed 1,500 m from the Pass of San Glorio in the west to the Pass of Los Tornos in the eastern part: Peña Labra, Castro Valnera and the mountain passes of Sejos, El Escudo and La Sía.
The rapidness of their waters, caused by their steep descents, gives them great erosive power, creating the narrow V-shaped valleys characteristic of Green Spain.
The environmental condition of the rivers is generally good, although increasing human activity due to rising population in the valleys continues to pose a challenge.
The rivers of Cantabria are short and rapid, descending steeply because the sea is so close to their source in the Cantabrian Mountains.