The Dam creation was finished more then 3000 years ago.
Predating the founding of the City itself.

The Dwarven King Filimohatant XV funded
its build as he began to realise it's mountains couldn't support it's people forever.

And while at first.
It seemed the fields beneath the Dam where rich in gemstones.

The damming began with a more controlled fish yielding.

It's soil that remained, appeared to be ideal for farming.

And gave rise to its first agricultural villages.