That’s not why the Panama Canal has locks. The locks are there for the purpose of lifting ships over the hump of land known as the Isthmus of Panama.
To get over that hump, the ships have to be floated upward 85 feet (26 meters) above the entrance ocean and then lowered back down into the exit ocean on the other side. That goes for both directions.
Why didn’t they just dig a flat ditch from one ocean to the other, a so-called sea-level canal? Mostly because it would have meant excavating a tremendous amount of dirt at tremendous expense. But also, there would have been torrents of water gushing through a sea-level canal. That’s not because of any permanent difference in level of the two oceans, however; their average sea levels are just about the same. It’s because of the tides.
At the Pacific end of the canal the tides can rise and fall as much as 28 feet (5.5 meters), whereas at the Atlantic end the tides vary by only about 2 feet (60 centimeters). So there would be periodic surges of water through the canal from the Pacific to the Atlantic, that is, from east to west.
Do you think I got that backward? Isn’t the Pacific Ocean at the western end of the canal? Nope. Because of the way the Isthmus of Panama snakes around, the Pacific entrance to the canal is 27 miles (43 kilometers) east of the Atlantic entrance. Check a map.
A ship passing through the Panama Canal from the Pacific Ocean to the Atlantic Ocean sails from east to west. (It’s actually southeast to northwest, if you must quibble.)
Now back to what we mean by “sea level.”
Obviously, because of the tides, we can talk only about the average, or mean, level of any ocean at any location. While the mean levels of the Atlantic and Pacific Oceans may be approximately the same at the Panama Canal, for example, that doesn’t mean that all the oceans in the world have settled down to a common level. You might expect them to be that way, because if you look at a globe you’ll see that they’re all connected; Earth’s oceans are one gigantic swimming pool with chunks of land scattered about. But even when you average out the tides, there are reasons why the oceans differ in their levels.
For one thing, because gravitational effects are bigger for bigger masses, the bigger oceans will be raised into higher tides by the moon’s gravitational pull. (Only the biggest lakes have tides.) Weather patterns also affect the sea levels. When the air pressure over an ocean is low, the water will actually expand.
Moreover, prevailing westerly winds can actually make the water pile up somewhat toward the east. And finally, differences in ocean depth can have a gravitational effect on the water’s level, because the deeper an ocean is, the more tightly its waters are packed down by gravity, and the lower its surface level will be.
These are all small effects, but when working on such huge bodies of water, they can make significant differences in the sea level at different locations in the world. Since they are all connected, the waters do try, of course, to seek a common level, but they are just too slow to keep up with all of these changing conditions.
So what is mean sea level? It’s a carefully compiled average, measured over a period of nineteen years at many tidal stages in many places around the world.
Whenever you hear that something is so many feet or meters above “sea level,” or that the atmospheric pressure at “sea level” is so many inches or millimeters of mercury, it’s understood that they’re talking about mean sea level: a worldwide, long-term average.