# St. Margaret's Bay Hydroelectric System

The St. Margaret's Bay Hydroelectric System consists of three hydroelectric power plants (two collocated in the same building) and the related lake, dam, and river systems driving them. 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. The system is part of the East Indian River watershed and drains 271 square kilometres (105 sq mi), with water storage provided in Lunenburg County by Mill Lake, Coon Pond, Sandy Lake, Wrights Lake, Big Indian Lake, Five Mile Lake and, at times, an overflow from Pockwock Lake in the Halifax Regional Municipality. The system is owned and operated by Nova Scotia Power Inc.
System assets associated with the St. Margaret's Bay Hydro System, which represents six generating units with an operating capacity of 10.2 MW, include Beeswanger Dam, Five Mile Dam and Wing Dam 4, Mack Lake Main Dam, Five Mile Wing Dams 1,2 and 3, Big Indian Dam, Wright's Lake Dam, Coon Pond Dam, Sandy Lake Dam, Sandy Lake and Coon Pond Pipeline, Mill Lake Plant and Surge Tank, Little Indian Crossover, Mill Lake Dam, Tidewater Plant and Surge Tank, Tidewater Pipeline, as well as associated headponds. Outflow is into St. Margarets Bay, Nova Scotia between Head of St. Margarets Bay and Tantallon.
Commissioned 8 June 1922, St. Margaret's Bay Hydroelectric System is the oldest hydro plant in Nova Scotia.

## Technical characteristics
### Dams, Wing Dams, Spillways, and Canals

There are 19 dams and wing dams of various heights, lengths and construction types holding back the lakes and reservoirs that make up the St. Margaret's Bay Hydroelectric System:
| Dam or other Structure | Height (ft.) | Length (ft.) | Foundation | Type |
| Coon Pond Dam and Spillway | 32 | 408 | Constructed on bedrock | Concrete & Earthfill |
| Sandy Lake Dam and Spillway | 50.5 | 1,021 | Constructed on bedrock | Concrete & Earthfill |
| Big Indian Spillway | 25.1 | 430 | Constructed on bedrock | Concrete |
| Big Indian Main Dam (Concrete) | 37.9 | 470 | Constructed on bedrock | Concrete |
| Big Indian Main Dam (Earthfill) | 33.3 | 120 | Constructed on bedrock | Concrete & Earthfill |
| Big Indian Wing Dam No. 1 | 4 | 75 | | |
| Big Indian Wing Dam No. 2 | 2.5 | 85 | | |
| Five Mile Lake Main Dam | 14.5 | 695 | Constructed on bedrock | Concrete |
| Mack Lake Dam | 20 | 410 | | Earthfill |
| Five Mile Lake Wing Dam No. 1 | 6 | 200 | Freeboard dam | Earthfill |
| Five Mile Lake Wing Dam No. 2 | 4 | 165 | Freeboard dam | Earthfill |
| Five Mile Lake Wing Dam No. 3 | 4 | 105 | Freeboard dam | Earthfill |
| Five Mile Lake Wing Dam No. 4 | 10 | 500 | Freeboard dam | Concrete |
| Beeswanger Dam | 8 | 460 | Freeboard dam | Earthfill |
| Wrights Lake Dam and spillway | 17 | 326 | | Concrete & Earthfill |
| Wrights Lake Wing Dam No. 1 | 4 | 20 | | |
| Wrights Lake Wing Dam No. 2 | 3.5 | 20 | | |
| Wrights Lake Wing Dam No. 3 | 5 | 35 | | |
| Little Indian Lake Cross-over Control Structure | 4 | 170 | Constructed on bedrock | Concrete & Earthfill |
| Mill Lake Head Pond Dam (Tidewater Plant) | 20 | 830 | Constructed on bedrock | Concrete & Earthfill |
Water for the system comes from Five Mile Lake, Big Indian Lake, Sandy Lake, Wrights Lake, Coon Pond, Mill Lake and, at times, overflow from Pockwock Lake. The two principal storage areas for the Sandy Lake Development are Five Mile Lake and Big Indian Lake.
On the Eastern side of the system, drainage into Wright's Lake and Coon Pond, in addition to surplus water from Pockwock Lake (owned by Halifax Regional Municipality), supplies the Mill Lake Plant. The new Coon Pond Dam has water continually flowing out from a maintenance chute to ensure water is always feeding a stream leading to St. Margaret's Bay to maintain fish habitat in the stream.
Output water from the Sandy Lake and Mill Lake plants then flow into the Mill Lake Head Pond to supply the Tidewater Plant which outputs at sea level into St. Margaret's Bay.
The total elevation from the highest reservoir to sea level is 134.2 m (440 ft). The St. Margaret's Bay Hydroelectric System has an annual output of 30 Gigawatt hours of electricity from two .6 MW generators at Mill Lake and two 2.9 MW generators at the Tidewater plant.

### Mill Lake Generating Station

Located on the Northeast River. Two vertical francis turbine units, shaft direct connected to the generators, with individual direct connected exciters, 49.5 m (162.4 ft) static head, 514 rpm, 1.42 MW (1,904 hp), for a total of 3.73 MW (5,000 hp). Two generators, 1,600 kVA each, 13,200 volts, 3 phase, 60 Hz. The original penstocks were built out of wood, using tongue & groove staves held together with metal bands (the same technology used in wood barrels), 1.82 m (6 ft) diameter, 965.6 m (3,168 ft) long, later replaced in part with fibreglass penstocks. More recently (2010) the penstocks from Coon Pond to the Mill Lake Station were replaced with black polypropylene, buried underground.

### Sandy Lake Generating Station

Sandy Lake Generating Station was added to the system in 1927. The power house was built beside the Mill Lake Plant. Two vertical francis turbine units, shaft direct connected to the generators, with motor generator set excitation, 38.1 m (125.0 ft) static head, 514 rpm, 1.86 MW (2,494 hp), for a total of 2.84 MW (3,810 hp). Two generators, 2,000 kVA each, 13,200 volts, 3 phase, 60 Hz. The original Penstocks were built out of wood tongue & groove staves with metal bands, 2.43 m (8 ft) diameter, 1,756.5 m (5,763 ft) long, later replaced in part with fibreglass penstocks.

### Tidewater Generating Station

Located on the shores of St. Margaret's Bay. Two vertical francis turbine units, shaft direct connected to the generators, with individual direct connected exciters, 27.9 m (91.5 ft) static head, 300 rpm, 2.57 MW (3,446 hp), for a total of 5.14 MW (6,890 hp). Two generators, 3,900 kVA each, 13,200 volts, 3 phase, 60 Hz. The original Penstocks were built out of wood tongue & groove staves with metal bands, 3 m (10 ft) diameter, 909.2 m (2,983 ft) long, later replaced in part with fibreglass penstocks.

## 2012 Upgrade

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.

## 2016 Surge Tank replacement

The original 43.9 m (144 ft) tall Sandy Lake Surge Tank was installed in 1927, and refurbished in 1996. In 2016 it was replaced with a new Surge Tank made of steel and fiberglass.

## Restoration of Fish Habitat

Gaspereau have returned to Indian River on the St. Margaret's Bay Hydro System for the first time in well over 100 years.
On 16 May 16, 1881, an inspector with the then federal Department of Marine and Fisheries toured the St. Margaret's Bay watershed and noted the lack of fish because dams used for running and milling logs did not have fish ladders.
In 2015 Nova Scotia Power invested $4 million into building the largest fish ladder in Nova Scotia at the Sandy Lake Dam.
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.
Pettipas was working on another project the day the biologist saw the three gaspereau. He headed out to the fish ladder the next day and dipped his underwater live view camera into the fishway. It turns out there were so many gaspereau he didn't need the camera.
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. Gaspereau swim up the river in the spring to spawn in the still waters of the lake then swim back to sea in the fall.
But it seems the new fish ladder, through both its design and the 30 cubic feet per second of water that flows through it, created enough white, bubbly water in the river to attract some of the fish to the ladder.
From the ladder, the fish will make their way to Sandy Lake.
Walmsley said this success holds tremendous potential. It's hoped the fish will eventually extend to the whole catchment area of the Indian River.