The system is situated within both Lunenburg County and the Halifax Regional Municipality, beginning approximately 2.5 km (1.6 mi) east of the Head of Saint Margarets Bay and approximately 10 km (6.2 mi) southwest of Hammonds Plains.

Commissioned 8 June 1922, St. Margaret's Bay Hydroelectric System is the oldest hydro plant in Nova Scotia.

[2] More recently (2010) the penstocks from Coon Pond to the Mill Lake Station were replaced with black polypropylene, buried underground.

The system was upgraded in 2012 with $17.8 million of work to replace the old wooden penstocks in fibreglass, a new water surge tank, a new dam at Mill Lake, a new gatehouse and gate at Mill Lake, and new floating booms to keep debris away from the dams.

[5] Gaspereau have returned to Indian River on the St. Margaret's Bay Hydro System for the first time in well over 100 years.

On 17 May 2016 – 135 years and one day after the federal inspector had been there - a field biologist with Nova Scotia Power checked the ladder and spotted three gaspereau in the resting pools.

He headed out to the fish ladder the next day and dipped his underwater live view camera into the fishway.

The intent of the ladder is to ensure the passage of species of fish that have to migrate to fulfill their life cycles, such as gaspereau and the endangered Southern Upland Atlantic salmon.