Spain and Portugal are lands of mines during the Roman period: gold in the northwest, copper and silver in the southwest, silver in the southeast. Quite a panoply of hydraulic machines are used to evacuate water from deep galleries in the peninsula’s Roman mines: Archimedes screws, Ctesibios pumps, water wheels. Water is also used to wash sediments so that heavy metals, like gold and silver, can be settled out and recovered. This is the classic technique of gold miners working on rivers, a technique that has come down to us from Antiquity. After having visited silver extraction installations in the region of Cartagena (Carthago Nova), Strabo writes:
“[…] as for the silver ore collected, […] it is broken up, and sifted through sieves over water; that what remains is to be again broken, and the water having been strained off, it is to be sifted and broken a third time. The dregs which remain after the fifth time are to be melted, and the lead being poured off, the silver is obtained pure.”[267]
The exploitation of Spain’s richness in gold is of high importance to the Roman Empire since gold is one of the bases of its currency. In the northwest of the peninsula, there are huge surficial deposits of gold-bearing sediments from ancient river deposits, moraines, etc. Enormous quantities of water are needed to wash these sediments and remove the slag, and likely also to wash down entire hillsides of gold-bearing alluvial sediments. The necessary water is brought to the mining sites through a network of aqueducts and canals, sometimes supplied by the capture of rivers, and sometimes by dams. The water is typically stored in large reservoirs next to the excavation sites. Pliny the Ancient, who visited some of these sites (in the second half of the 1st century AD), was visibly impressed:
“Another labour, too, quite equal to this, and one which entails even greater expense, is that of bringing rivers from the more elevated mountain heights, a distance in many instances of one hundred milles perhaps, for the purpose of washing these debris. [….] The fall, for instance, must be steep, that the water may be precipitated, so to say, rather than flow; and it is in this manner that it is brought from the most elevated points. Then, too, valleys and crevasses have to be united by the aid of aqueducts, […] The water, too, is considered in an unfit state for washing, if the current of the river carries any mud along with it. The kind of earth that yields this mud is known as “urium;” and hence it is that in tracing out these channels, they carry the water over beds of silex or pebbles, and carefully avoid this urium. When they have reached the head of the fall, at the very brow of the mountain, reservoirs are hollowed out, a couple of hundred feet (c. 60 m) in length and breadth, and some ten feet (3 m) in depth. In these reservoirs there are generally five sluices left, about three feet (1 m) square; so that, the moment the reservoir is filled, the floodgates are struck away, and the torrent bursts forth with such a degree of violence as to roll onwards any fragments of rock which may obstruct its passage.”[268]
Remains of this system have been discovered in a region of the present province of Leon called La Valduerna. Claude Domergue describes a reservoir 170 m long, with a variable width from 33 m to 70 m and a maximum depth of 2.8 m, separated from upstream to downstream into two compartments of 3,500 m3 and 9,825 m3 in volume. Another reservoir of more than 10,000 m3 has been found in the same region, above a circular deposit some 2 km in diameter. Additional reservoirs that are more than 250 m long[269] have been found on the cliffs overlooking the mining sites.
