Ska is a torrent running in the mountains of Kato Nafpaktia. It flows between mount Rigani (1469m) and the complex of peaks on the northwest of Nafpaktos, which rise just above 1000m., such as Prophitis Ilias, Paliokastro, Koutra and Tsokana. Its ever flowing springs must be sought high on the mountainous ravines, but at lower altitudes its riverbed is dry and full of stones. However, when it rains constantly, large amounts of water flow making it brash and dangerous.
Its springs are located in the start of two large ravines neat the villages of Niokastro and Ano Mamoulada. At this location you will have the chance to see continuous, impressive waterfalls that flow in the cavities created by the erosion. A bit higher up we meet a stone fortress that stands from the Venetian period. Since the old times, the canyon was a passage towards the mountains of Venetiko and Kravaras.  Nowadays, this path is accessible and ideal for a hiking expedition. In the western sides of the canyon you will see the historic church of Agios Ioannis that can be accessed from the village of Vomvokos. In the eastern sides, you will encounter an archaeological site that can be accessed only on foot from the village of Skala or through a path in the canyon.
Source: http://www.agro-tour.net/