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St.Margarets Bay Hydro-electricity Plants, Green Renewable Power Since 1922

First I would like to thank Nova Scotia Power for helping my research, including  a tour of the two hydro-electricity plants serving St.Margarets Bay, Nova Scotia.



St .Margaret's Bay Water System & Catchment Area

System Modernisation

How It Works

 Environmental Preservation

Tidewater Hydro-electric Plant


The tremendous increases in demand for power during the early 1900’s for the Halifax Metropolitan area could not be met by the antiquated thermal units which in use at that time.  Rates and prices being paid for power by the city became very high, underlining the need for a cheaper source of electricity. In addition, the high capital cost of any power development of sufficient magnitude to cover the new demand, made it necessary to have the backing of the provincial Government.Studies recommended to the province of Nova Scotia the development of two sites at St.Margaret’s Bay.  It was determined that three potential power sites existed in the area, and that the construction of two of these should be started at once.The Mill Lake plant, located on the Northeast River, and the Tidewater plant on the shores of St. Margaret’s Bay, were to provide the city of Halifax, Nova Scotia with 20 Million kWh of power per year. When the Sandy Lake plant was finished some time in the future, it was expected to make an additional ten million kWh per year available.

With the help of the consulting firm of C.H. & P.H. Mitchell, of Toronto, Canada, the Commission finalised plans for the development of the sites, and tenders were called early in 1920. Actual work began in May of that year, with little knowledge of what the final cost would be. Over the next four years, more than $1,700,000 was spent on construction of the two plants.

At the time of construction, there was much doubt about how the new system would operate. The public had very little knowledge of electricity and their fears caused much hesitancy. On hearing, that power lines with 13,000 volts passing through them were to be built, farmers between St. Margaret’s Bay & Halifax, Nova Scotia expressed their fears that the electricity would leak into the ground, killing vegetation and livestock.

Throughout 1920 and 1921, the Nova Scotia Power Commission continued work on the two sites. By July 1921, general work, being done by the contracting firm D.G. Loomis & Sons was completed, and commission crews took over the remainder of the work.During 1921, a great many discussions had been held between the Power Commission and the city of Halifax, to negotiate details of a contract whereby the city might utilise power from the St .Margaret's Bay development. However serious doubts about the merits of the development arose, and a group of citizens decided to ask for outside help. In a letter to Sir Adam Beck, Chairperson of the Ontario Hydro Electric Commission, these citizens solicited a study by an expert from Ontario. Mr. J.H. Jeffrey was delegated by Sir Adam Beck, and travelled through Nova Scotia making a detailed study.In a report published in the evening mail to the citizens of Halifax, Nova Scotia, Mr. Jeffrey stated “ the plant has been well designed and constructed along modern lines...... The St.Margaret’s Bay development should give the city of Halifax aThe other main point of contention was the ownership of the system. Many citizens thought that it should not be owned by a private corporation, since rates might then be raised for profits.

very reliable power service, in fact quite as reliable as cold be obtained from the most modern stream plant”.

Four options were possible:-

After a lot of discussion, the city decided that such a venture could prove too risky, and refused to either buy the system, or buy power from the Commission. The Tramways Company, which had earlier dismissed the development as foolhardy, jumped at the chance to buy power and and distribute it within the city. Thus finally, a formal contract for the delivery of power was signed in February of 1922 with the Nova Scotia Tramways and Power Company Limited, and with the completion of transmission lines to Armdale,  8th June 1922 marked the day when power was first sold to Nova Scotia Tramways and subsequently to the people of Nova Scotia.In 1927, the sandy Lake Plant was added to the system when the power house was built beside the Mill Lake Plant. This addition increased the systems total power capacity of 13,010 horse power and supplied about 30,000,000 kWh annually to homes in Halifax, Nova Scotia .

Mill Lake Hydro-electric Plant

Modern Automated Control Rooms

The Energising of the line from St.Margaret’s Bay to Halifax, Nova Scotia was the first large scale operation undertaken by the commission, and its tremendous success has set the tone of work ever since. The St.Margaret’s Bay system was, for many years after, the showcase of hydro-electric installations in the Atlantic Provinces. The St.Margaret’s Bay development demonstrated that hydro-electric power could be generated profitably in the Nova Scotia, and paved the way for larger operations in the future.

St .Margaret's Bay Water System & Catchment Area

The water supply for the St .Margaret's Bay system drains from an area of about 86 square miles (222 square kilometres). The two principal storage areas for 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, supplies the Mill Lake Plant.

Water from Sandy Lake Plant and mill Lake Plant flows into Mill Lake Head pond which is the water source for the Tidewater Plant.

View Larger Photo of  St.Margarets Bay Water System & Catchment Area

System Modernisation

The first supervisory control board- a system which uses push-button operations instead of manual operations at Nova Scotia Power was installed in 1958 in the Mill Lake Plant. This allowed plant operators to start the turbines, program the controller and carry out general management of the Tidewater Plant on the shore of St .Margaret's Bay from the Mill Lake Plant. This reduced the need for operators to work at the Tidewater plant as well.

In the mid 1970's, continual development, technological advancements and reorganization within the Corporation replaced approximately 25 employees at the system reducing that number to five in the mid 1990s. Today (2011), all plants are automatically controlled and have self-protection systems. When problems occur on any part of the system, operations immediately shut down until maintenance employees are able to correct the problem, there are currently three regular employees who manage and operate the whole St .Margaret's Bay hydro-electric system.The St .Margaret's Bay system was the first of all Nova Scotia Power’s energy-generating systems to install fibreglass penstocks, replacing the parts of the original wooden penstocks. Penstocks are the pipes that carry water from the dams to the turbines and can be as large as 11 feet in diameter.

In 2010 the penstock from Coon Pond was replaced with polypropylene buried underground.

Old Style Gauges - no longer in use

How It Works

Mill Lake

(head pond)

Control Dam

Level of water

Surge tower


St. Margaret’s Bay


Power House


Francis Turbine

Head - 91.5 feet

Above sea level

Diagram showing how the St. Margaret's Bay Hydro-electricity plant works


The St. Margaret’s Bay  Power Plants use the Francis Turbine which can operate with 90% efficiency, the turbine is the part that is turned by the water passing over the blades, which in turn rotates the generator.

Top Of Turbine At The Mill Lake Plant

view of  the head pond (Mill Lake) from Control dam  looking towards the Mill Lake Power Plant

Head Pond

The water starts at the head pond where the levels & flow of water to the turbines are controlled by the dam.

Head Pond


The water leaves the control dam down through the penstocks, which are large pipes of up to 11 feet in diameter towards the turbines located on the shore of St. Margaret’s Bay.The original Penstocks were built out of wood 2a), using tongue & groove staves held together with metal bands the same technology as wooden barrels.

The St .Margaret's Bay hydro-system was the first in Canada to use fibreglass penstocks which can be seen in photo 2b). More recently (2010) the  penstocks from Coon Pond to the Mill Lake Plant were replaced with black polypropylene, which although buried underground can be seen in photo 2b)

2a)Original Wooden Penstocks

2b)Modern Fibreglass & Polypropylene penstocks


Surge Towers

The Surge Tower is connected to the penstocks located near the power house and is a very important part of the St. Margaret’s Bay Hydro-electricity system.

The surge tower has three important  functions:-

Surge towers To The Mill Lake Plant

Surge Tower To The Tidewater Plant

Surge Towers



The Generator is the business end of the hydro-electric system and is rotated by the turbine below, which  produces the electricity to be sent to the main grid to be used by the consumer.

Environmental Preservation

The low impact of the hydro-electric plant is shown here by the active nesting of Ospreys

Nova Scotia Power’s concern for the environment has resulted in the preservation and enhancement of wildlife habitats in the ponds and around the plants and dams of the hydro-electric system.

At the head ponds, where water is reserved, trout fishing is popular, Eagles continue to nest along the shores of Coon Pond, one of the head ponds beside Mill Lake. Ospreys are seen fishing in the tailraces to feed their young.

In winter, ducks, muskrats and mink are also found feeding in the ice free water below the stations.

More recently Nova Scotia Power have built an Osprey nesting platform beside the Mill Lake Plant, which as can be seen in the photos is being used by the Ospreys.

Osprey Nesting Platform



What is this?

head pond penstocks surge towers turbines generators

Samuel  W. Gray Civil Engineer St.Margarets Bay Hydro system

Many thanks to John S Gray, Grandson of Samuel W Gray for sending me these photos of the Mill lake Power plant which were taken circa 1931.

Mill lake power Plant Circa 1931

Samuel W Gray inspecting the turbines destined for the Mill Lake Power Plant circa 1931

Thanks to Sandy Wooden for sending these photos taken 29th October 1920 .

Sandy’s dad George Nash started work for the power Commission in 1935 and retired as District Supervisor of the St.Margarets Bay System in 1974

Left Photo - Oak Penstock from Mill Lake

Middle Photo - Granite Tailrace Tidewater

Right Photo - Construction Of Penstock