The rivers of Cantabria are short and rapid, descending steeply because the sea is so close to their source in the Cantabrian Mountains. They flow perpendicular to the coastline, except for the Ebro. They also generally flow year round due to constant rainfall. Nevertheless, the rate of flow is modest (20 m³/s annual average) compared to the other rivers of the Iberian peninsula. 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.