It captures the high rainfall in the catchment of the King River and allows diversion of water through a tunnel to the John Butters Power Station downstream of the dam.

This had never been successfully attempted before in the design of dams of any significant height, due to problems in making allowance for embankment settlements.

The spillway is designed to allow sufficient time for a large jet flow valve located in the diversion tunnel to be opened so that larger floods can be safely handled.

[8] The dam is named in honour of James Crotty who founded the North Mount Lyell Copper Mine at the turn of the 20th century.

In the 1910s the Mount Lyell Mining & Railway Company had investigated and surveyed a site very close to this dam for a proposed hydro electric scheme.