Cachão da Valeira was a waterfall in a rocky gorge of the Douro River with a water current that fell from a reasonable height, situated upstream from the mouth of the River Tua and about 6 Km from it.
In this place of legends and dangers, the first attempts to clear the river of total navigability were made in 1530.
But it was only in the 18th century, during the reigns of Dom Pedro II and Dom João V that various studies were carried out for the construction work. In 1779, the Companhia Geral da Agricultura e Vinhas do Alto Douro was authorized by Queen Maria I to collect taxes on the wine, brandy and vinegar that were transported on the River Douro, with the aim of investing them in works that would make it navigable.
Father António Manuel Camelo, from S. João da Pesqueira, was in charge of destroying the rocks and widening the riverbed, assisted by José Maria Yola, a hydraulic engineer from Sardenha. Thus began the work of demolishing Cachão in 1780, with many hundreds of explosions given below the waterline and thus widening the riverbed.
The first boats started going up and down the river in 1789 and in 1792 the work was declared completed. Before, the boats would sail up the river from the mouth of the Douro, in Vila Nova de Gaia, only as far as Cachão, which prevented them from navigating beyond it upstream.
In 1976, the Valeira dam was built. This dam includes a lock with a 33m drop that allows cruise ships and other vessels to pass through.
An inscription in Cachão da Valeira, which dates back to the 18th century, is in place at Ermo, on the left bank of the River Douro, about 1 km upstream from the Valeira dam, which can only be reached by boat. It is an epigraphic inscription inlaid on the north-facing face of a rock with Latin, capital letters, inlaid with gilded bronze and topped by the inlay of a closed crown. In it is mentioned, among other things, the glorious achievement that allowed the development of the entire region.