Post Falls: Powering Northern Idaho for More than a Century

The Post Falls hydroelectric project on the Spokane River in northern Idaho began operating in 1906 to provide power for local industries. The plant continues to generate electricity today, using the same equipment installed more than 100 years ago. Post Falls is this year’s Hydro Hall of Fame inductee.

By Michele M. Drake

In northwestern Idaho, about 9 miles west of Couer de’Alene Lake, the Spokane River splits into three channels, each with a natural falls. Just downstream of the falls, the three channels rejoin. In the late 1800s, pioneer Frederick Post discovered this site and immediately recognized its development potential. He secured rights to the site from the Coeur d’Alene Tribe of Indians and built a sawmill. He also built a dam across the river’s north channel to divert flow to his sawmill. The timber-crib dam was about 20 feet high and several logs thick. Historic photographs show the dam was built using layers of long logs, connected with layers of short logs. The dam did not have head gates, so water flowed over the entire dam during periods of high flow.

In 1902, the mill was destroyed by fire; the dam was undamaged.

Two years later, the newly formed Washington Water Power Company purchased Post’s site and began making plans to develop a hydroelectric facility. The company had already built a hydro plant, Monroe Street, in the city of Spokane, Wash.

The new hydro plant at Post Falls would provide power to mines nearly 100 miles away via the longest high-voltage transmission line in the world.

The company’s development plan for the site was ambitious. The plan called for construction of a dam across each of the three channels at the site. The powerhouse would be constructed in the middle channel, which was the deepest and most confined of the three, with the highest head for energy production. A dam in the middle channel would form the upstream wall of the powerhouse. The dams across the north and south channels would control spill in periods of high flow and maintain water levels in the forebay during times of low flow.

The wooden dam Post had erected at the site was so well-constructed that Washington Water Power used it as a cofferdam when building the north channel dam. Construction crews bolted the dam’s base to natural rock and to a substructure and filled it with concrete to make a watertight connection to the bedrock channel. The north channel dam features an “L”-shaped spillway with eight 21-foot-wide sluiceways running east and west and one 110-foot-wide sluiceway running north and south, perpendicular to the main river channel.

Once the north channel dam was completed, Washington Water Power turned to the powerhouse and middle and south channel dams.

The middle channel dam – 431 feet long and 31 feet tall – forms the upstream wall of the brick and mortar powerhouse. The 175-foot-long powerhouse was built to house six turbine-generating units. However, only five units, each with a capacity of 2.25 MW, were initially installed – the first three in 1906, the fourth in 1907, and the fifth in 1908. The utility anticipated installing the sixth unit at a later date, when demand increased. (A sixth unit was added in the 1980s, increasing the plant’s capacity to 14.75 MW.)

The dam in the south channel of the river is 78 feet long, with six 13-foot-high, 6-foot-wide gates through which the utility could pass peak flows.

Together, the dams form a 40,600-acre reservoir with the capacity to store 223,100 acre-feet of water. Figure 1 on page 26 shows the site layout.

Washington Water Power completed the three dams and the powerhouse in only two years, no small feat considering that construction technology was limited to horses, manpower, and ingenuity.

Innovation was commonplace during construction. For example, to deliver lumber for forms, carry loads of concrete, lift head gates, and complete other tasks to build the dam, construction crews erected towers and used a trolley system. The tower and trolley system enabled Washington Water Power to build the powerhouse in the middle channel of the river, while allowing for the free passage of water through the north and south channels during spring runoff.

In another example of innovation, Washington Water Power built a 60-kilovolt (kV) transmission line from its generating plants in Spokane to the construction site. This line provided a constant, reliable source of electrical power, eliminating the need to rely solely on small engines, boilers, and long lines of steam pipes to provide power for construction activities. This arrangement allowed the company to greatly increase the rate of construction, even during freezing weather.

To combat freezing temperatures during the winter months, construction crews used a steam boiler to provide hot water for mixing concrete and piped steam under tarps to help the concrete set.

Since its completion, the Post Falls project has served as an important source of electricity for the operation of mines, mills, factories, cities, businesses, and railways in northern Idaho and eastern Washington.

Present-day Post Falls

In the early 1990s, to enhance recreational offerings at the site of the Post Falls facility, a citizens’ group in the community of Post Falls, Idaho, built Falls Park along the north channel of the Spokane River. Washington Water Power (known as Avista Corporation since 1999) donated 22 acres of land for the park, which includes picnic areas and playgrounds, scenic overlooks of the project’s spillway and powerhouse, and wheelchair-accessible trails. Interpretive signs placed throughout the park describe area history and features. Development of the park was financed through private donations and grants.

Click here to enlarge image

On a recent spring day, the park was full of visitors. A father and son navigated a small radio-controlled car, children enjoyed the playground, and visitors strolled along the trails and bridges. In this period of spring flooding, most people made their way to the far edge of the park to take in the splendor of water thundering through the spillways of the century-old dams and over the 50-foot-high natural falls. There, conversation seemed to stop as visitors paused to simply take in the power of the water plunging into the gorge below.

A century-old portfolio

In addition to Post Falls, Avista Corporation owns and operates two other century-old hydroelectric projects: Monroe Street and Nine Mile.

The 14.82-MW Monroe Street project holds the distinction of the longest operating hydro development in the state of Washington. Washington Water Power built the project in 1890. In 1972, the utility completed a major rebuild and rehabilitation of the project, and in 1992 a new underground powerhouse and generator were installed.

The 26-MW Nine Mile project was completed in 1908. The project initially was constructed to operate an electric railway in the area. Washington Water Power purchased the project in 1925. Between 1928 and 1930, the company built a number of brick cottages to house personnel working at the project. Today, both the hydro plant and the cottages are listed on the National Register of Historic Places. n

Ms. Drake may be reached at Avista Corporation, P.O. Box 3727 MSC-1, Spokane, WA 99220; (1) 509-495-8941; E-mail:

Michele Drake is compliance coordinator, environmental affairs, for Avista Corporation, owner of the Post Falls plant.

Technical Information

Post Falls

General Information
Location: Spokane River in Idaho
Owner: Avista Corporation
Capacity: 14.75 MW
Rated Head: 50 feet
Flow: 5,400 cubic feet per second
Average Annual Generation: 88,500 megawatt-hours
On-Line Date: July 1906

Turbines (6)
Double horizontal Francis
Five installed 1906 to 1908:
2.25 MW
3,260 horsepower (hp)
138.5 revolutions per minute (rpm)
Manufactured by Platte Iron Works
One installed in 1980:
3.5 MW
5,500 hp
164 rpm
Manufactured by Allis Chalmers
Generators (6)
Five installed 1906 to 1908:
2,250 kilovolt amperes (kVa)
1.0 power factor
2,300 volts, three phase
Manufactured by General Electric
One installed in 1980:
4,375 kVa
0.8 power factor
4,375 volts, three phase
Manufactured by General Electric

North Channel Dam
Concrete gravity
31 feet high, 431 feet long
Eight tainter gates, one rolling sector gate
Middle Channel Dam
Concrete gravity
64 feet high, 215 feet long
Eight vertical lift head gates
South Channel Dam
Concrete gravity
25 feet high, 127 feet long
Six steel vertical sluice gates
Brick and mortar
175 feet long, 83 feet wide
Penstocks (6)
Riveted steel
11.25 feet in diameter
56 feet long

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