Penstocks take water from 1,000 feet down.
The power plant discharges its water into the Flatiron reservoir.
Granby is also part of the Colorado Big Thompson Project.

The Flatiron Hydroelectric Plant is stationed at Flatiron Reservoir and is one of several facilities used by the Colorado-Big Thompson Project – a large water diversion project designed to collect West Slope mountain water and divert it to Colorado’s plains.

In April 2009 it was announced that the plant had been awarded £14m from President Barack Obama’s stimulus package. The money will be spent on recoating 50-year-old pipe linings.

Plant history

The feasibility study for the plant was carried out in 1937 and it first went into operation in 1954. In total, the Colorado-Big Thompson Project consists of over 100 structures that together form a water diversion system that spreads over 250 miles.

The Flatiron plant generates electricity for the project and provides water to Carter Lake for delivery to other customers.

Plant details

Flatiron uses a conventional pump generator and a Francis Turbine to obtain its total generating capacity of 94,500kW – increased from the original capacity of 71,000kW. The plant contains two main power units and a reversible 13,000 horsepower pump turbine unit. The power plant discharges its water into the Flatiron reservoir, which is regulated to provide controlled water release into the foothills storage and distribution system.

“Flatiron uses a conventional pump generator and a Francis Turbine to obtain its total generating capacity of 94,500kW.”

Flatiron uses a reversible pump to lift water from the Flatiron Reservoir – the maximum lift needed being 297 feet. Water is then taken through the Carter Lake 1.4 mile pressure conduit and tunnel. When this action is reversed, the mechanical structure (Unit 3) acts as a turbine-generator and produces electricity. This happens in periods of high demand where water is released from Carter Lake to flow back to Flatiron reservoir and 8,500kW of electricity is generated.

Unit 3 is capable of discharging a maximum of 370 cubic feet per second into Carter Lake and operates on surplus or off-peak power generated by the project’s other power units.

Water discharged from the power plant is then kept in Flatiron Dam, which is used for after bay storage. Gravity then takes it north through the Charles Hansen Feed Canal, across the Big Thompson River.

New developments

As part of the American Recovery and Reinvestment Act introduced by President Barack Obama during the financial crisis, the Colorado-Big Thompson Project was awarded $14m and work will now begin upgrading Flatiron by the US Department of the Interior Bureau of Reclamation.

According to a report in the Colorado’s Reporter Herald, the Bureau plans to hire a team to replace existing coatings on the 6-ft wide pipes that carry water from the project to the power plant, and were originally installed in 1962.

“The Colorado-Big Thompson Project was awarded $14m.”

The large pipes, known as penstocks, connect the plant to the Colorado-Big Thompson project to the hydroelectric plant via Bald Mountain. They are either used to generate electricity or to provide water for 720,000 residents. The linings and exterior of the pipelines, which drop 1,064 down the mountain, will be recoated in an upgraded product than that originally used in the 1960s.

All previous lead-based paint and packing that may contain asbestos will be removed and new packing added where necessary. In addition, workers will install ultrasonic flow meters to improve water accounting and power plant efficiency.

It is thought that work will be carried out during the quiet months and no power supply to customers should be affected. This project will bring jobs to Northern Colorado as well as reduce long-term operational costs and increase efficiency and reliability by extending the pipes’ service life an additional 50 years.


  • Start of operation – 1954
  • Output – 94,500kW
  • Plant type – Hydroelectric
  • Location – Colorado
  • Estimated Investment – N/A (new developments $14m)
  • Project management – Bureau of Reclamation