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In sci-fi and fantasy artwork, I often see a city built on top of or incorporating waterfalls:

enter image description here

James Gurney's Dinotopia (above) is an egregious example with "waterfall towers". When the city is underground, there's often a central waterfall pouring into a lake. And in the film Logan's Run (below) they escape the city through a postmodern water sculpture park – it's purpose isn't explained.

enter image description here

Is there a utilitarian excuse for artificial waterfalls in a (mostly) realistic fantasy or sci-fi setting?

I'm looking for plausible infrastructure, sanitation, irrigation, or atmospheric purpose that would justify the expense and maintenance of channeling artificial waterways through a city, or deliberately building a megastructure city on top of natural water channels and cliffs.

A watermill in every home does not sound like plausible infrastructure – even in a medieval watermill-punk world. I can't imagine that thousands of localized water turbines (even Tesla turbines) would be better than a large centralized hydroelectric powerplant. (Maybe I am wrong?)

How can I justify a City of Waterfalls trope to serve a plausible utilitarian purpose?

As suggested by the two photos, the waterfalls don't need to be impressively large or in any particular configuration, just open-air and scattered throughout the structures. I want the form to follow function – whatever that is. My setting is sci-fi/fantasy (no magic, not Earth, all human) with refugees living around repurposed heavy industry, so my "waterfalls" need to serve a utilitarian function (at one time if not currently).

I am specifically not looking for aesthetics or "because they can" reasons, such as the waterfall skyscraper in China, rather a utilitarian or infrastructure purpose.

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24 Answers 24

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Low-tech climate control: the waterfalls fill the air with spray, which evaporates off and cools the populace.

The Romans used to achieve a similar effect with fountains.

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    $\begingroup$ So effective, it's still used in many cities in southern Europe. In the summertime heat, plazas with fountains in them are noticeably cooler than the surrounding streets. $\endgroup$ – Nuclear Wang Dec 10 '18 at 13:57
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    $\begingroup$ @NuclearWang not only Southern, it works all right in Poland, too! $\endgroup$ – Mołot Dec 10 '18 at 14:03
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    $\begingroup$ Keep in mind that water is very absorbent in the infrared spectrum. The higher the air humidity the more heat the air "holds". So if there is not enough draft to move that moisture away, the effect might be the opposite and create a hot house. I think fountains in sci-fi are mostly used because they are "cool" in the figurative sense. Simple aesthetics. $\endgroup$ – dtech Dec 11 '18 at 9:32
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    $\begingroup$ @Mołot we Poles need to remember this perspective knowyourmeme.com/memes/you-forgot-poland ;-) I've learnt it myself bringing up Poland as an example in some answer/comment. Of course you're right and the principle works everywhere, including Poland. $\endgroup$ – Ister Dec 11 '18 at 9:33
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    $\begingroup$ @dtech because water vapour is lighter than air, it creates draft if there isn't already. (Which is also part of the reason why fountains cool.) Only in a full enclosure would the greenhouse-effect of water vapour invert the cooling effect, but in that case you'd probably have a literal greenhouse anyway... $\endgroup$ – leftaroundabout Dec 16 '18 at 15:05
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Still water is heaven for proliferating algae, bacteria and annoying insects like mosquitoes.

A large city with large bodies of still water is a recipe for plague spreading. Having water on the move keeps the water clean and, as a consequence, the city healtier.

Also, waterfalls help enriching the water with oxygen, helping bacteria to decompose organic matter suspended in it, again improving cleanliness.

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    $\begingroup$ IIRC, moving oxygen-rich water is also a great for aquaponics. $\endgroup$ – user10328 Dec 10 '18 at 17:00
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    $\begingroup$ If you had a city with troublesome open sewers, but could not dig to put them underground, you could incorporate waterfalls to flush the sewers and turn them into streams. $\endgroup$ – Willk Dec 11 '18 at 0:35
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    $\begingroup$ @Willk nobody wants to live in the city of poo falls. $\endgroup$ – Captain Man Dec 11 '18 at 15:09
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    $\begingroup$ @Chronocidal: why do you not want to live in Choleradonia? $\endgroup$ – smci Dec 12 '18 at 10:56
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    $\begingroup$ I would much rather live in the city of poo falls, than the city of poo lake. I'd also much rather live in the city of poo falls than the City Below Poofalls... $\endgroup$ – Haakon Løtveit Dec 12 '18 at 11:43
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Energy storage.

You use a form of energy generation with unreliable or periodic output. As a form of energy storage you pump water up into a nice big reservoir when you have surplus, then power turbines by letting the water spill back down to a lower reservoir to create a more steady power supply.

It might be more efficient in an enclosed system of pipes, but for aesthetic reasons and public morale it’s much nicer to have exposed waterfalls in public spaces. As @PluckedKiwi pointed out in the comments, the presence of additional waterfalls/ absence of usual waterfalls can act as a very public indicator of current energy supplies.

It’s pretty simple, pretty reliable and pretty.

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    $\begingroup$ As a bonus you have a great visual and auditory signal to all residents that it is a time of plentiful energy, so they should conduct any energy-intensive activities while the waterfalls are going. If the waterfalls are not apparent, city is on restricted power consumption and everyone should be attempting to conserve power. $\endgroup$ – pluckedkiwi Dec 10 '18 at 13:50
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    $\begingroup$ Except the waterfalls are fairly useless for that, the visibly falling water is wasted height/energy. You can only capture energy from the water that falls into a turbine. $\endgroup$ – Harper - Reinstate Monica Dec 10 '18 at 19:19
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    $\begingroup$ @pluckedkiwi that’s the wrong way round; the water is pumped up during “plentiful” electricity consumption, and released when they need more electricity. And plentiful is a poor descriptor; electricity is produced precisely as needed. $\endgroup$ – Tim Dec 10 '18 at 20:41
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    $\begingroup$ @JoeBloggs that is flat wrong. $\endgroup$ – Harper - Reinstate Monica Dec 10 '18 at 21:15
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    $\begingroup$ @JoeBloggs However (AIUI) you only get energy from the length of the pipe. Once the water escapes into free air, you don't get to extract energy from the rest of the drop since it doesn't generate suction. So it's better to have the pipe go all the way down to where the waterfall would end, instead of having the pipe go halfway down and then having a waterfall. $\endgroup$ – user253751 Dec 11 '18 at 2:24
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You're looking at it backwards. Nobody added artificial waterways to a city. They did what they had to do to support building a city on top of existing waterways.

Human settlements naturally spring up near water sources for a number of different reasons. An annoying drawback of these water sources is that they limit the settlement's ability to expand. When the settlement evolves into a city and you need more room, you can't simply move the water source out of the way (unless you're Dutch, of course). In your image, the cliffs add another limit to how far you can expand your city.

You're trying to make the most use of the land available so you've clearly built your city all the way out to the cliff edges and to the very banks of the river. Rivers are notoriously restless things, though. They meander all over the map and when they run over a cliff edge, they slowly erode away the cliff. Neither of these are particularly good things when you have roads and buildings in very close proximity.

A relatively cheap and easy solution is to constrain the river so that its course is fixed. Line the river channel with concrete to prevent meandering, and design outlets at the cliffs that prevent erosion. Add artificial channels with multiple outflow points to increase your water throughput, which helps protect your city against river flooding.

You'll end up with a large-scale erosion control project where you've carefully replaced the river's natural bed and channel with erosion-resistant material, and where you carefully monitor and control water levels, diverting water as necessary to avoid flooding. To an outside observer that didn't see your city before the project, it looks like you built a bunch of waterfalls for some reason.

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    $\begingroup$ Adding to this, you could also have gradual landmass rise cause the sea below to retreat from the city, allowing more structures to be built further down the cliff, and adding height to the original waterfalls. $\endgroup$ – lime Dec 11 '18 at 3:18
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    $\begingroup$ I wonder if you are refering to Bad Gastein in Austria, which fits your description precisely! $\endgroup$ – Pavel Dec 11 '18 at 11:22
  • $\begingroup$ @Pavel - I wasn't, but I agree it does fit the description. And a gorgeous photo as well! $\endgroup$ – bta Dec 14 '18 at 1:20
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Waterfall is power

In Shannara Chronicles castle of Humans was built on what looks like hydroelectric dam:

Frame from video

Both having electricity (or other kind of energy if it's fantasy) and being able to dry or flood large terrain is pretty utilitary, and nothing shows it off better and is foolproof more than simple waterfall - it keeps maximum level of water in reservoir without any moving parts that could break.

Graduation towers

If you have a salt mine nearby, waterfall can be used to increase salt in solution, an important step in salt purification. Sure, most common design is based on wooden sticks, but regular waterfall requires far less maintenance.

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  • $\begingroup$ Or Pitlochry, Scotland, though the town was at the end of the loch long before the hydro dam was built. $\endgroup$ – Pete Kirkham Dec 13 '18 at 18:03
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refugees living around repurposed heavy industry, so my "waterfalls" need to serve a utilitarian function (at one time if not currently).

A. Diverting a major waterfall for mining

Vital minerals were found under and on the vertical face of a large natural waterfall (Niagara etc.) In order to mine them, a method of diverting the water away was invented that didn't require changing the course of the whole river. That was impossible for large-scale geographical/geological reasons.

The old civilisation simply built chutes outwards from the top of the existing falls. Eventually the outward aspect of the mine became huge and more and extended chutes were added at different levels. People who lived at the mine workings had their own personal chute to keep their house or front door dry. This all proliferated in a haphazard and ad hoc way.

B. Sluice boxes - panning for gold

Where the two mighty rivers meet the silt is churned up and gold can be found. A powerful family and their thralls took over the whole of the ramshackle mining town that had grown up around the confluence.

There are many sluices fed by the river that filter out the gold.

The powerful family gradually built a beautiful city on their great wealth and the gold-collecting sluices were made more and more beautiful as well as retaining their function. You could literally dip your hand in one and have a handful of gold.

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If staged and planned right artificial waterfalls can increase the sound quality to an area. Flowing water can dampen sounds that are undesirable and create curtains of white noise to block out anything that could be considered undesirable as noise in an area. I've worked construction jobs before where if you had a river or waterfall near by you wouldn't hear as much construction depending on which side of the water you were on.

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  • $\begingroup$ Probably not that useful (how much construction do you expect to have?), but certainly an interesting effect. $\endgroup$ – Draco18s Dec 10 '18 at 15:08
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    $\begingroup$ I was using the construction as an example but you could use it to block "industrial" noise from a manufacturing district from effecting a residential district. $\endgroup$ – Michael H. Dec 10 '18 at 22:46
  • $\begingroup$ That makes more sense. $\endgroup$ – Draco18s Dec 10 '18 at 23:23
  • $\begingroup$ Water was used this way for the space shuttle: nasa.gov/mission_pages/shuttle/launch/… $\endgroup$ – Logan Pickup Dec 11 '18 at 9:39
  • $\begingroup$ @LoganPickup It was also used to absorb the energy so the resulting shockwave didn't destroy the engine bells. $\endgroup$ – Stese Dec 13 '18 at 13:23
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There are several purposes which you can fulfill with a structure of channels and waterfalls. On the one hand, channels within your city provide a fast lane means of transportation in any age of human technology. As L.Dutch pointed out, keeping the water moving instead of standing still keeps bacteria and algae from developing into serious health problems for your citizens. They can also provide either a source of fresh water or a sanitation possibility (more plausible in a low tech setting) for your citizens. Definitely, if have water channels and even more so, if you have water falls, you are provided with a natural way of micro-climate regulation. Cities tends to heat up in summer time/during heat periods, which water can compensate providing a cooling effect and keep the air from becoming too dry to make infections of the respiratory system more likely. In addition, the aesthetic effect of water channels and park lanes or avenues alongside it should not be underestimated - it keeps your citizens happy alongside more efficient infrastructural uses. The platforms above water falls can also make attractions for the city's more advantaged citizens/nobility or possibly every citizen and demonstrates the power of their ruler to be able to build such an amazing structure. In addition, these large waterfall structure can actually have the turbines of your city-wide power plants at their bottoms without limiting the effect of any of the advantages listed above.

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Hydathodes

hydathodes

https://www.botany.one/2017/07/hooray-for-the-hydathode/

Plants sometimes can build up internal water pressure. The water needs somewhere to go or it could burst the tissues. Some leaves have special pores called hydathodes. These pores release excess water in a process called guttation.

So too your city. Underneath there is excess water pressure. It must be released in a controlled fashion or it will invade buildings and ruin roads. The hydathode towers allow the pressure to be contained and released.

Bonus: Hydathode is a rocking fantasy name for a city.

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    $\begingroup$ Squirt City, Spurtopolis, Pore Haven. Dewville... hmmmmmm. $\endgroup$ – wetcircuit Dec 11 '18 at 3:28
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Your city gets its water from nearby mountains, however due to the proximity and difference in height the water arrives at very high pressure, too high for the primitive pipes of the time to handle. Since the city does not have valves and other control systems more importantly part of the city needs that high pressure (water wheels). The simplest way to bring the pressure back down so the water can be sent to other districts is a waterfall which releases all the pressure at once in effect resetting the pressure gradient and bringing it back down to a manageable level.

But the city is clever the pressure is used to drive a water wheel system, overshot water wheels in which the water comes from above are much more powerful than undershots ones. So they have row/stacks of water wheels powering various mills around the city, but you cant push all the water through wheels it is too unpredictable and will wreck tour mills. so excess water simply goes an alternate route and falls (see image). The city designer was smart enough to realize leaving the waterfall design allows for the city to build more mills and wheels later, allows the mill wheels to be well controlled, and lets the same water be sent to homes, irrigation, ect. You will see many falls staggering downward to generate different pressures for different purposes, high pressure for irrigation, low for drinking water, ect.

Niagara falls used to have a massive mill complex, but the mills could never use even a fraction of the water available there was just too much. later tourism became more profitable than the unsightly mills. If a city had been built around both the unsightly mills might never have disappeared.

enter image description here

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    $\begingroup$ Your comment about needing to reduce the pressure for home delivery reminded me of the distribution towers in Pompeii (as always, look to history and you will likely find something of the sort). Think of it as a water tower, but with an open top. The water coming from higher elevations comes up like a fountain, releasing all its pressure as it fills the box at the top of the tower, which is drained by other pipes going elsewhere only pressurized by the height of the distribution tower. $\endgroup$ – pluckedkiwi Dec 10 '18 at 15:49
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You're mentioning heavy industry, so in addition to already mentioned climate control a very strong and useful function of waterfalls would be

Reducing pollution

Tiny water droplets that are included in a mist are capable of capturing and holding dust and pollution fine particles. Then being heavy they fall down on earth and are eventually washed to sewer system. As the droplets small enough are not that easy to distribute over the city, as the city grows the mist generators (namely waterfalls) are added.

In modern times China uses this technique, however instead of waterfalls they have special mist cannons.

The mist cannon, which is not well-known but gaining popularity, is a large machine mounted on a truck that sprays water mist. (...)

"The truck works by nebulizing water into tiny particles, with diameters in microns, the same as PM2.5 and PM10, and spraying them into the air, and then they could combine with the dust and fine particles to fall to the ground," Xu said. (...)

According to tests conducted by researchers with the China National Environmental Monitoring Center, the mist cannon and the chemicals1 can together "cut the PM10 by 20 percent, PM2.5 by 5 percent and nitrogen oxides by 10 percent", according to Xu.

There are two trucks covering 35 streets


1The chemicals are used to capture pollution fallen to streets, mostly from cars.

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  • $\begingroup$ Beat me to it! The other part of this is to help lay the dust, to help wash dirt out of the air. $\endgroup$ – Criggie Dec 12 '18 at 0:53
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Defensive purposes:
The city is on water, but cannot be attacked on water, because the the waterfall is making an almost unescapeable stream. The city is also granting very less option for an ground attack, because of such small bridges, which are even destroyable or which can be pulled in like castle bridges.
If the bridges are completely destroyed the city is only accessable to creatures(/machines) which can fly or can resist a waterfall stream.
That makes good military bases too.

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The answers so far are awesome. They talk about energy/power, water, air quality, aesthetics, and warfare. There is a line about "transportation". Mining. The assertion that the waterfalls were there and they did what they had to.

But I could add a few others ....

Your pictures show overshoot waterfalls, which allow a reservoir behind the falls that is higher than the water in front of the falls. Given the Gibbs free energy surface of water, that is to say, given that water bonds have high energy, and nature goes to the lowest energy state, there is a preferential movement of stuff in the water to the surface. Alcohol does this. It could be a mechanism for separating things that are somewhat soluble (think stills and moonshine), but also have a lower Gibbs free energy surface, like an open-air refinery and storage.

Ponds like that could also be about sky-cooling. (It is one of my favorite techniques, and a technology lost to the modern world). This guy is selling silicon, so he contrives his answers only in terms of silicon. Silicon is not required. He asked the genii of his mind a weaker question, and received a weaker solution.

Water isn't always liquid. It freezes. It evaporates. What happens when the freezing cold of winter hits those styles of falls. At least half the volume is stored as liquid behind the (very insulating) ice of the frozen fall. It could be about retaining liquid, possibly retaining biodiversity such as aquaculture fish, or magical protozoa, or such. Humid air is heavier than normal air, so the pit-like region could be about retaining humid air.

There could be a half-and-half where the "humid" "air" forms mists, or micro-clouds, that have a purpose.

Water is made of hydrogen and oxygen. Perhaps it feeds an oxygen converter? Fusion power-plant? Deuterium-oxide (heavy-water) or tritium-oxide (very heavy water) mining for nuclear-energy uses?

It could be used to transport oxygen. Modern fish-tanks have waterfalls that entrain and dissolve oxygen so the fish can live.

The blocks at the lower part of Logans run are for turbulating (and slowing down) rushing floodwaters. Rivers don't always run at the same height. When there is rain upstream they run high. Perhaps this is about enduring the larger climate instead of the local comfort. Perhaps, like the cathedral under Tokyo (link), they help the city to endure natural disasters.

I like to think about things like chemically pumped lasers. Ever hear of the Jello-laser? What if it is carrying particulates of metastable lasant through a beam-line? You would need something about the waterfall to not poison the metastable state, and you would need a long enough metastable lifetime. The "COIL", chemical oxygen iodine laser, had long metastable lifetimes. I often wondered if it was a dual-use technology, because the iodine could be used in the 3rd stage of fission-fusion-fission nuclear pumped lasers as part of a 3-stage nuclear weapon, or even a star-wars-esque nuclear pumped super-laser for engaging a ground-based target with mega-scale laser irradiation. If there was a material with a long-enough metastable lifetime, the civilization could be filtering it from the water, from a far past war, filtering it from the water to power their infrastructure, or rebuild the terrible ancient weapons.

I thought about this for a long time, but I think there may be waves in the oceans that have been moving since the big bang, that carry some of that information. I have also thought that if there were two places that had a "structural weakness" in the universe where it would be easier/easiest to make a worm-hole, and one of them was at an ocean, the waves would show it. An ocean that is a billion years old, could have a billion years for the gravitational or inertial perturbations of the wormhole-end collected in a way that is encoded in the waves. I have thought about the Nikola-Tesla + Philadelphia experiment, and wondered if Tesla's inverse free electron laser, meant to absorb radar for radar-scale invisibility, by oscillating the ship at the frequency of the radars, stumbled onto such a phenomena. By inverted, I think he turned a FEL inside out, so the beam became like a surface on the outside of the ship. If an ocean had those Tesla-Philadelphia waves, then a super-intelligent race might use a waterfall in the ocean to try and distill primal oscillations in order to build doorways to other worlds, or other dimensions, or to measure far-away parts of the universe remotely.

It could be about plumbing. Dinosaurs make a big poops.

I've seen pneumatic logic circuits. It could be a hydraulic variation, where it is performing logic or computations. If it were the right material, it could be a quantum computation. See slide 41 of this to show macro-scale quantum-mechanics (top row of table) connects Heisenberg scale to Kolmogorov scale, its on the order of 10 microns, or about 1% of a millimeter, but who is to say that a large-scale structure running a surface of such quantum computations would not look like a waterfall.

In a world at war with marine creatures, they could be used through giant pipes to make very loud, very specific wavelength, underwater sounds, like active sonar, which devastates current cetaceans. It would be like a giant pipe-organ, for water, that makes sounds that fend off war-parties. You could make sounds that are loud above ground too.

Could they power hydraulic and/or steam cannons? Water powered resets of siege warfare equipment. Can we make a water-driven reset of a trebuchet? We could make ice rocks to shoot, or ice arrows to shoot. It could provide both ammunition and motive power.

I lived in the Sonoran desert for 16 years (Phoenix), and one thing that happens every time it rains is desert spores reproduce like mad. I had a friend who was allergic, and when they lived downwind of a football field, they had alergy attacks every time the field was watered. The waterfall could be about aerosolizing water, to trigger molds or other organisms.

Update (Jan 2019):

  • They could act like a giant "bong" in filtering part of an aerosolized "medication" so that the remainder was a higher quality in some way.
  • If the civilization had a "infinite cold source" then it could be used to turn liquid water into freezing clouds of ice. I've thought about it some, but there are going to be optimal crystal configurations for ice-replication. There have been sci-fi stores about building ice-format that self-replicate and "destroy the world" into an ice-ball. If they exist, those are encoded at a microscopic scale in the axes and shapes of snowflakes. I think you could make micro-snowflakes with great Stokes numbers (so they stay in the air for a long time) with surface roughness/smoothness such that they are very effective reflectors of light, so they change albedo. I could see something like this as a doomsday weapon against a world, to turn it into ice by injecting enough of those crystals, at a high enough rate, to destroy the world, and make it fight against recovery - to make it resist ability to sustain life forever. It might take simple oscillators and flow directors up near the top, and the right kind of blower a little farther down. I've wondered if China/Mongolia could make something like this that injects into the jet-stream, and turns Alaska into an iceball while making a solid ice-bridge at the Bering strait. How long does that area have to be kept at 40-below before the ice gets "deep enough"? A counter-waterfall could "drown out" or have chiral anti-snowflakes that either reduce below critical concentration, or selectively target and capture the problem flakes during interactions. This would allow two competing massive waterfall/snowmakers to fight it out, where one sustains a warmer climate, and the other tries for ice-ball.
  • If they wanted to a civilization with giant acoustics, and existing clouds, might want to try and induce the super-thermal-radiant surface topology into the top of the cloud layers, so they might make very particular acoustics to make non-snowflake based ice-doomsday weapons against the world.
  • Converting gravitational potential energy into heat for some warmth increase. (link) In a high gravity world, the potential heating effects are much higher. There is an interesting effect that the center of the planet is weightless. There is a linear decrease in "weight" as you go toward center. If you wanted to stake a "Krypton" energy source, then redirect the gravity waves, and put huge but arguably sustainable crust stresses on the entire world in order to make gravitational potential energy into a "hotspot" for energy extraction. Gravitational wave teleportation? Can gravitons tunnel? Can they be induced into spatial (or temporal, because they are the same thing and the scanning-tunneling microscope is really time-travel too) tunneling?
  • Machining. One could use it as a giant tumbler. If you did it right, you could concentrate the pressure (think about how waterjets make mach 3 water) and use it for cutting, for pressing, for bending, for "peening"
  • I have been thinking about the meta-meaning behind myths, and how the deep past can strongly speak, in much more meaningful and powerful ways that science can, to the present and future. This is in the sense of Jordan Peterson, such as here. The waterfall could speak something of the deep past to the deep future in a meta-meaning sense. Fruit trees are important to human development, and so something like this could apply.
  • If we think about a "living planet" like the organ equivalent of the difference between an endo-skeleton and an exo-skeleton such as grasshopper, then water carries nutrients and acts like hemolymph or blood. It could be argued that the air currents are a second type of "blood". If the water is blood for the inside-out massive meta-organism then all that blood is, the water is. Transport. Immune system. Where life of the cells comes from. In this paradigm, a waterfall is where the two different "breeds" of "blood" interact. For an organism, where two fluids interact, there are very specialized tissues, and there is immune activity to prevent propagation of disease. Not only can water transform the gas, but he gas can transform the water. In an organism substantially more advanced than an insect, the lymph nodes are where blood and lymph mix and interact - they are a critical part of the immune system. One could argue that rocks, for a living planet, are like bones. So the waterfall is air and water and stone - blood and lymph and bone.
  • I like Brian Deragon's answer, but we could do a bit of both. What if there was flaming oil on top of water? Coal, when heated, can burn underwater (link). What if there was submerged burning substance as well as floating ones? Defense? Offense? Disposal of waste?
  • When I look at the waterfall, I think the sea is lower. What if it isn't. There are dikes in the Netherlands, and when they leak it isn't a good thing. What if it isn't about using something that is going away to somewhere else (the water in the river) but is instead about something scary coming close? What if the dam is about the coming ocean, not the leaving river?
  • Appeal to the unknown: when you (as a technical nerd) want to solve an otherwise impossible problem, you know the center of your domain isn't where it will work, so you go to the half-lit edges. You don't go to the darkness, but to the shadow-areas. It could be that this is the imagination-equivalent of a shadow area in that, psychologically speaking, the presence of these style of waterfalls accelerates suspension of disbelief toward futuristic fictional universes. It might be a generational or cultural thing, but it might be a universal psychological mechanism.
  • I recently watched the pilot episode of "Dragon Pilot: Hisone and Masotan". When we think about cooling, there was a sense of "air-conditioning" but this could be a way to cool off a dragon. To cool some highly exothermic, semi-aquatic lifeforms.
  • Salmon leap waterfalls to get to their breeding grounds. Those who don't make it, don't breed. It could act as a form of natural selection for salmon or other aquatic organisms. It is also a route for fresh baby salmon to get to the ocean, so instead of reduction it could be a protected transport corridor that typical in-ocean predators cannot overcome. It doesn't have to be for salmon, but could be for tiny things or for huge things.
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  • Cooling the streets
  • Releasing negative ions - used in office artificial waterfall's and other high stress area's.
  • Irrigation
  • Waste Removal ( sewerage )
  • Climate control
  • Radio wave manipulation ( water effects radio waves somehow )

Not the most concise answer but i just wanted to add the main reason we use them at the moment - Negative ions

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  • $\begingroup$ "Not the most concise answer", we can see that you are quite new to WB.SE ;-) Consider taking the tour. And do not hesitate to flesh up your answer. $\endgroup$ – clem steredenn Dec 11 '18 at 8:45
  • $\begingroup$ Glad you added sewerage. Every water treatment plant I've visited has an aerator that looks a little like a fountain. $\endgroup$ – Walter Mitty Dec 11 '18 at 11:28
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    $\begingroup$ Negative ions? What have anions got to do with stress? $\endgroup$ – wizzwizz4 Dec 14 '18 at 17:27
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Defense

Did you ever try to climb a wall below a waterfall?

Waterfalls can help defend your cities in the following ways:

  • If the waterfalls are designed well enough, they could fall on areas that are likely to be used by enemies (or dangerous animals?) to enter the city.

  • The spray and subsequent mist helps hide the city. So it's harder for enemies to count the number of guards and spot weaker areas.

  • If a waterfall is falling in front of a door, it could be a cheap way to keep animals that fear water out of the city, while human will just get wet when entering. It is also harder for people entering to conceal weapons under wet clothes.

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    $\begingroup$ Fey or vampires, in legends, have some issues with crossing running water. $\endgroup$ – EngrStudent - Reinstate Monica Jan 3 at 12:54
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You don't even need to reach into the realms of sci-fi or fantasy for this. Some modern cities have artificial waterfalls installed in recreational areas that serve the utilitarian purpose of masking the city noise.

GreenAcre Park in New York City is one such example.

A city that contends with exceptional levels of noise might employ such on a large scale.

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You could also work it into your story, by allowing the water to be exchanged for other substances substantially more dangerous, such as burning oil. Then it would not only be an aesthetic choice, but also a defensive choice.

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    $\begingroup$ Scaling a wall is hard, scaling a waterfall is harder. So by itself it could also serve a defensive purpose on the outer city walls. $\endgroup$ – Spoki0 Dec 11 '18 at 9:46
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Although the plausibility can be debated, practitioners of Feng Shui believe that flowing water in the home will bring prosperity.

As a result, many Chinese families who have the means will place a fountain or a waterfall prominently in their home or garden, often with goldfish swimming in the pool below.

The influence of such beliefs might be one reason why a city would choose to build these kinds of features (appropriately placed of course). And those features might have a plausible benefit, if they attract other believers to the city to work or just to visit.

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They fill wells.

That picture from Logan's Run looks like an ancient public well I read about - in the Middle East, I think. It fills up during the rainy season. In the rest of the year, people come to the lowest step that's still above the water surface, and fill their jugs. As the water level lowers, the people have to go down to the next step down to reach the water, and so on.

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I'm focusing my gaze on an artificial waterfall something like 5 miles from here. I live in a desert, some years back they basically banned decorative water features other than in a few cases where they are substantial tourist attractions. Yet this waterfall was built after the ban....

What it really is is a cooling tower. Such things are usually a bit on the ugly side and placed out of sight, they simply got fancy with it and made a fully functional cooling tower that looks like a waterfall.

Your artificial waterfalls are being used to cool the buildings underneath.

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The waterfalls are a byproduct of what your civilization wished to achieve. What they really want is to have a deep lake to hide and protect their treasures and have to do something with the excess water.

Long ago, the ancient kings found that tomb raiders could not be effectively deterred so they began building their tombs in deep valleys and diverting a nearby river to flood them. Of course, the minor nobility soon followed suit so they began building their own, shallower tombs resulting in smaller waterfalls

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  • $\begingroup$ This has interesting implications when the dams "silt up". You could have Pompeii-like lost tombs that were hundreds of feet deep, but were built into the flood-plains. In thousands of years when the dam disintegrated, and erosion took over again, it could be revealed to sunlight. $\endgroup$ – EngrStudent - Reinstate Monica Jan 3 at 21:39
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Beauty

You ask for utilitarian purposes but I would argue that art and beauty are utilitarian. Every human culture has some form of art going as far back to cave paintings. I would argue that art is a necessity of human culture. The fact that your culture expresses that through architectural waterfalls does not mean that they are somehow unimportant in spite of not filling some other pragmatic purpose.

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In addition to all of the other answers, it can also prevent ships, either attackers or people escaping, from traveling on the water. This can help keep the place protected from attack

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The waterfalls are white noise generators.

The city contains industries that produce obnoxious clangs, whines, and buzzes. Or stockyards with unruly creatures, flies, and mosquitoes. The sound of the waterfalls masks these noises enough to let people think and sleep.

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