Where Does All the Water Go? The Surprising Waterflow Secrets of Niagara Falls

A rendition of the secret water highway hidden 140 metres (460 feet) beneath the city. This 12.7-metre (42-foot) wide tunnel
diverts a massive volume of the Niagara River to power 160,000 homes.

Every night at 10 p.m., while the world is sleeping, someone "turns down" Niagara Falls. It’s one of the best-kept secrets of this natural wonder. If you’ve ever stood at the edge and felt the roar of the mist, you weren't just looking at a waterfall—you were looking at a carefully engineered binational powerhouse.

Let’s dive into the numbers, the massive tunnels hidden underground, and the natural forces that keep this icon roaring.

Journey Behind the Falls
Niagara Falls, ON Photo by ©Sofistikateit

1. Mind-Blowing Volume: How Much Water Flows?

• The "Olympic" Speed: During peak tourist hours, the law requires a minimum of 2,832 cubic metres per second—which is approximately 100,000 cubic feet per second (cfs)—to flow over the brink. That is enough to fill an Olympic-sized swimming pool in exactly one second!
  
  
• A Weighty Reality: On heavy runoff days, the falls hurl over 2,870 metric tonnes of water every single second—that is approximately 3,160 short tons, which is the massive volume featured on our Niagara wall art! To put it in perspective, it’s like dropping 500 elephants over the cliff every time you blink!
  
  
• The 10 p.m. Change: Once the tourists go home, the "tap" is partially closed. A 1950 treaty allows the volume to be cut in half to just 1,416 cubic metres per second (approx. 50,000 cfs), diverting the rest to spin massive power turbines downstream.

2. The Nightly “Disappearing” Act

Have you ever noticed the Falls look slightly different at 2 a.m. than they do at 2 p.m.? It isn't just the lighting. To balance world-class tourism with clean energy production, a 1950 U.S.–Canada treaty allows power companies to "turn down" the Falls after dark and during the winter off-season.

By re-routing the river into massive underground tunnels, engineers can capture the water's energy when fewer people are watching. High-powered spotlights help keep the "slimmer curtain" looking dramatic for the few night owls still at the brink, even when the volume is significantly lower.

The Treaty Requirements:

• Tourist Hours (Apr 1 – Oct 31, 7 a.m.–10 p.m.): The treaty mandates a minimum flow of 2,832 m3/s (100,000 cfs) to ensure the "unbroken curtain" of water visitors expect.
  
  
• Non-Tourist & Overnight Hours: The flow is permitted to drop to 1,416 m3/s (50,000 cfs), allowing the rest to be diverted for power generation.

3. Powering Two Nations

Diverted water spins turbines in two giant plants:

• Sir Adam Beck Complex (Canada)
• Robert Moses Niagara Power Project (USA)
• Total Capacity: Roughly 4.9 gigawatts—enough to light about 3.8 million homes when both stations are running at full capacity.

4. How the Diversion Works

1. Intake gates upstream skim water off the river.

2. The water plunges almost 90 metres (approx. 300 feet) down into deep, rock-cut tunnels.

Cross-section infographic of
the Niagara Falls hydroelectric
diversion system

3. It then races through roughly 10 kilometres (approx. 6 miles) of underground tunnels to reach the hydro stations.

4. Finally, the water plunges through massive penstock pipes (think elevator-shaft-sized pressure tubes) that strike the turbines with immense force to generate electricity.

5. Erosion—Slowing Nature’s Sculptor

Historically, the Horseshoe Falls "chewed" its way upstream at a staggering rate of roughly 1 metre (3 feet) every year. At that pace, the Falls would have eventually eroded back so far that the water would have eventually drained out of Lake Erie!

However, thanks to the massive water-regulation treaty we discussed and various rock-stabilization projects, that pace has dropped to about 0.3 metres (1 foot) per year. This roughly three-to-one slowdown ensures that the majestic view we see today will be preserved for centuries to come.

6. Quick Facts

• Deepest plunge pool: 55 metres (180 feet) below Horseshoe Falls.
• Oldest exposed rocks at the base: approx. 450 million years (Queenston Shale).
• Winter ice boom: a floating steel-pontoon barrier at Lake Erie’s outlet keeps giant ice sheets out of the intakes.

7. Where Does It End Up?

Over the brink, through the Whirlpool Rapids, into Lake Ontario, down the St. Lawrence River, and finally out to the Atlantic Ocean.

8. Visiting Tip: How to Catch the Best Rainbows!

You don't need to be a scientist to find the best colors—you just need to keep the sun at your back and the mist in front of you.

• The Horseshoe Rainbow (Canadian Side): For the brightest, most vibrant arcs over the Horseshoe Falls, aim for the late morning to early afternoon (10 a.m. – 2 p.m.).
• The American and Bridal Veil View: If you’re looking toward the U.S. side, the colors tend to peak in the mid-afternoon as the sun shifts into the western sky.
• The Perfect Conditions: For the most wow factor, visit on a clear or partly sunny day.
• Spring Bonus: The rainbows are even more vibrant in the spring, when the heavy water runoff creates a much thicker mist for the light to play with.

More Than Just a Pretty Face

Niagara Falls isn’t just eye candy; it’s a binational powerhouse that fuels homes, sculpts ancient rock, and inspires awe. Next time you’re there, you’ll know exactly where all that water goes—and why so much of it is quietly at work behind the scenes.

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Resources:

International Niagara Board of Control. Flow summaries & treaty data. (IJC.org).

U.S.–Canada. 1950 Niagara River Water Diversion Treaty.

Ontario Power Generation. Sir Adam Beck Generating Stations fact sheet.

New York Power Authority. Robert Moses Niagara Power Plant profile.

U.S. Army Corps of Engineers & Environment Canada. Hydrology reports.

Geological Survey of Canada. Stratigraphy of the Niagara Escarpment.

Government of Canada. Canada-United States Niagara River Treaty.