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Like most tunnels, you can’t really see the extent of the hydro tunnels at Niagara Falls. There are a few conspicuous clues though, like these gigantic buildings. These interesting protrusions from the landscape house enormous steel doors, nearly 60 feet tall, that can drop down into the tunnels and close off the flow for inspections and maintenance. Both the Ontario and New York sides of the river feature similar structures.

From the tunnels, water flows into major hydropower plants on both sides of the border: the twin Adam Beck stations on the Canadian side and Robert Moses station on the US side. Then it’s released into the the lower part of the river below the falls. When you add them up, that’s 39 turbines with a combined capacity of more than 4000 megawatts. It’s a tremendous amount of power generation in one place. But actually, that’s not all of it.

All of this diversion noticeably reduces the flow of water over the falls. Even when they are at ‘full blast’ during the day in the tourist season, only 50% of the flow of the Niagara River makes it over the falls. You can imagine how powerful the falls would be if 100% of the flow were to cascade over. It might seem like this diversion detracts from the majesty of the falls, but in another sense, it actually preserves it.

After the engineers got a chance to inspect the situation, they determined that the best course of action was just to leave the majority of the talus in place, since it seemed to be stabilizing the cliff face. Sometimes, doing mostly nothing is a decision you make as an engineer, even if you have to do a monumental amount of work to come to that conclusion. So the cofferdam was taken out, and water has flowed continuously over all the falls since then.

Niagara Falls is one of the most spectacular waterfalls in the world. With a vertical drop of more than 50 meters or 164 feet and a flow rate that often exceeds 2800 cubic meters per second or 100,000 cubic feet per second, it’s one of North America’s crown jewels. Roughly ten million people visit the falls every year just to catch a glimpse of the curtains of water pouring over the edge and the constant clouds of mist at the bottom. But Niagara Falls isn’t just a tourist attraction. The special geology and hydrology of this region, situated between Lake Erie and Lake Ontario, have resulted in some fascinating feats of infrastructure, from shipping to electricity to water control. It’s basically a microcosm of all the things I love. The falls themselves have required quite a bit of engineering over the years, and they’ve even been shut off for maintenance. Let’s take a little tour of the Niagara Peninsula (even though it’s really an isthmus), and I’ll show you some of the things that aren’t usually listed in a guidebook. I’m Grady, and this is Practical Engineering.

Let’s get oriented first. This is a map of the isthmus. We’ve got Lake Erie to the south, Lake Ontario to the north, Buffalo and western New York to the East, and Ontario, Canada, to the west. The Niagara River runs northward, connecting the two great lakes. And right in the middle, it plunges off the Niagara Escarpment, creating the famous falls. On the US side, there are the American Falls and the smaller Bridal Veil Falls. And on the Canadian side is the Horseshoe Falls where a majority of the river flows. It’s pretty impressive to see in person, but it’s actually not entirely a benefit. Because these falls pose a major problem for shipping.

The Great Lakes form the largest inland freshwater transportation system in the world. Since the 19th century, they’ve served as the backbone for moving iron ore, coal, grain, and manufactured goods between the American heartland and the Atlantic Ocean. Ore from Minnesota and grain from the Midwest can travel by ship all the way to steel mills or export terminals on the East Coast. Barges and freighters are efficient at moving bulk cargo in a way rail and trucks can’t match. For a time, the Niagara Escarpment was a natural bottleneck between Lake Erie and Lake Ontario, preventing goods from moving directly between the upper lakes and the Atlantic. Freight had to be offloaded and portaged around the falls before it could continue its journey. The Erie Canal solved the problem somewhat, starting in 1825, bypassing Lake Ontario. But it could only accommodate smaller vessels, and even before the Canal opened, another solution was being planned.

The Welland Canal runs through the peninsula west of the Niagara River, connecting two massive areas by shipping traffic for the first time in 1829. The canal fueled the early growth of cities along the Great Lakes and St. Lawrence River – including Cleveland, Detroit, Milwaukee, Chicago, Toronto, Montreal, and Quebec City – and it’s been rebuilt and moved several times over its life. The Welland Canal is really a titanic engineering achievement and, were it not positioned next to one of the natural wonders of the world, it would probably be famous in its own right. Because of the huge difference in elevation between the two lakes created by the escarpment, eight separate locks are required to allow ships to traverse between them. And all different kinds do – from personal leisure craft to the lakers that stay in fresh water to the salties that travel between the lakes and the ocean through the St. Lawrence Seaway.

Starting on the upstream, Lake Erie side of the canal, the first lock isn’t really for lifting or lowering ships so much as for control. The level of Lake Erie actually fluctuates throughout the year, and there are longer-term trends as well. Wind storms also raise the level locally similar to the way storm surge works during hurricanes. The control lock does just that: it controls the level in the downstream canal. It prevents excess water from rushing down the canal when the lake is high, kind of like an airlock on a spaceship keeps air from rushing out when astronauts step outside for a spacewalk.

Downstream of the control lock, the canal splits in two. The original pathway of the canal flows through the eponymous town of Welland, while the larger and newer section of canal, the Welland Bypass… well, it bypasses Welland to the east. If you look carefully, you’ll also notice a small river, the Welland River, which passes underneath both the original and bypass canals. On the way downstream from Lake Erie to Lake Ontario, shipping traffic passes over aqueducts that pass over a natural river. A hydrological wonderland!

Continuing downstream from the aqueducts, the remaining seven locks are lift locks, more like what you think of when you imagine a lock. Notice how they’re clustered tightly around the terrain and not distributed evenly along the length of the canal. That’s the Niagara escarpment, the same geological feature that the water cascades down at the falls. This is the elevation diagram of the entire Great Lakes and St. Lawrence Seaway system from Lake Superior to the Atlantic Ocean, and you can see that this drop is the biggest one of the whole thing. And that’s pretty important for another part of the infrastructure on the peninsula.

The power available from a moving fluid is directly proportional to the flow rate multiplied by the height of the drop. In most hydropower applications, that height is created artificially by a dam. There aren’t that many places in the world where you have both a large volume of flowing water and a significant natural drop in elevation. But that combination made Niagara Falls the birthplace of large-scale electric power in North America. In 1895, the Niagara Power Company opened the Edward Dean Adams Power Plant, built with Westinghouse AC generators based on the ideas and patents of Nikola Tesla. The plant served as the basis for the modern electrical grids we have today, and many of the fundamental concepts are basically unchanged.

These tunnels divert 50-75% of the flow of the Niagara River. That wide range in percentage of diversion isn’t because we don’t know how much is diverted, but because we actually control how much water is diverted, depending on the tourist requirements agreed upon in a treaty by both nations. During the day in peak tourist season, more water is allowed to flow over the falls to ensure the grandeur of the falls is on full display for the huge crowds of tourists that visit every year. At night and during the winter, more of the flow is diverted to generate power. That’s all managed by this structure upstream of the falls: the international control dam.

I’ve always thought this is an interesting dam, since it doesn’t even go all the way across the river. But it doesn’t need to. This structure’s not meant to create a reservoir; it just subtly adjusts the level in the river to control how much water flows over the falls versus into the hydropower intakes. The US side of the Niagara River is pretty shallow, so that side acts kind of like an uncontrolled spillway. Then, the gates on the Canadian side can be adjusted to balance the competing demands on water between tourism and power.

But there’s one big problem with those competing needs: they both have the same timing. We want thunderous cascades of water over the falls during the day when tourists are visiting, but daytime is also when the demand for electricity is highest. It’s like if solar panels only worked at night. To accommodate this, both the US and Canada have pumped storage plants. At night, excess electricity is used to pump diverted water into reservoirs, essentially storing both the power and the extra water that’s available during off-peak hours. Then, during the day, the water is released back into the forebay of the power plants. You get a little extra power from that drop out of the reservoir into the forebays, so both sides have small hydropower facilities to capture that. But more importantly, you get a lot more water during the day than would otherwise be available to run through the big plants, making more power when it’s needed most. And there’s just something funny to me that the infrastructure is duplicated on both sides of the river, like neither country was willing to be one-upped by the other.