In the mid 1800's, the threat of fire forced Ottawa and it's surrounding towns to create a better infrastructure for water supply in the area. Before the construction of the pumping station, water was only supplied by individual wells, which would have been of little use against a fire.
In 1875 the Fleet Street Pumping Station was opened in Lebreton Flats, Ottawa. It used the natural currents of water to transport it across the city with little electricity. By channeling water from a tunnel at Chaudiere Falls, a turbine at the site was powered and used to pump water into the city.
Since then, two additional water pumps have opened at Lemieux Island and Britannia, which provide the city with the majority of it's filtered water. However, the pump at Fleet Street is still in operation and is the only one of it's kind in Ontario.
From what I've read on UER, the channels below can be entered during the winter and other times when the water is low, but I have never tried personally. The majority of the facility has been fenced off due to construction this past year, but I recall when I was younger one could walk up to the doors and windows and peer inside. It is also open during doors open Ottawa every year.