Great Dam (Zoroaster)

A great dam collects runnoff water from the mountains and leads it down to the plains below, irrigating areas that lack rivers.

The great dam must be built in a mountain province, and forms an artificial riverhead from which canals can then be extended.

Note that this is not the type of aqueduct built by the Romans - this type of artificial river is of much greater capacity, but does not carry the water in such a dramatic fashion. It gathers water in a huge dam and then forms an artificial riverbed, with only occasional bridges over ravines and such.