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Comment by HarHarVeryFunny | Hacker News Reader

Why would gravity require a water supply other than what's already in the canal?

o To go up you sail in, close the gate behind you, and let water into the lock from the higher level until you can open that gate and sail out

o To go down you sail in, close the gate behind you, and let water out until it has dropped to the lower level and you can open that gate and sail out

At, least, this is how normal canals/locks work.

What's special about the Panana canal ?

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If you don't use pumps you need your whole supply of water to be sourced from the hight of the high point in the locks. How would you propose to operate a pure gravity lock system without an outside supply of water?

With the Panama canal, the crucial difference is that a large part of the canal is Lake Gatún, a freshwater lake, so they can't pump water up to avoid overdrawing the water supply

There has to be water coming in from higher up than the highest point in the lock system. Otherwise, how do you fill up the lock when you're traveling to the top? In a "normal" canal, this is handled by the upstream river or stream. In the Panama canal, this is accomplished from two lakes above the canal system.

It's multiple locks, first up, then down. The middle section is at a higher altitude than the ocean on both sides

I believe that the high point is in the middle. So in either direction, you have to climb, then descend, and when you open the last gate all that water goes into the ocean.

I think you're missing that, when you go down, the water flows out into the lower level waterway, not into a holding tank. It therefore cannot be re-used to raise the level in the same lock again. Water goes from the upper level to the lower level only. So you need a supply of water at the upper level.

What I was missing is that in the Panama canal the high point is in the middle, not at one end, so the water flow is from middle to both ends.

In a normal lock system (e.g. typical British canal) the water flow is from the high end of the entire set of locks to the low end, so gravity does the job, and the water supply at the high end is replenished by whatever it is connected to (river or lake).

also, the lakes in the middle are freshwater, so pumping water back in from the sea would destroy the lake's ecosystem and any sort of supply hooked up to it expecting fresh water, which is not in the interest of the Panamanians.

> let water into the lock from the higher level

ok, what about the highest level of the lock?

A big lake in the middle

Both situations involve a huge amount of water flowing downstream. That needs to be replenished either through rainfall or through pumping.

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