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The "city on a bridge" that I'm envisioning has the entire population living their entire lives on a habitable bridge or a series of such bridges over major rivers, only going on land for recreation or for work-related reasons (farming, hunting, logging, mining, etc.). All housing and businesses that don't need to be on land would be on the bridges. Travel within the city would mostly be by foot, bike, or public transit (light rail along the bridge, ferries between bridges).

My thought process here is, once a civilization has the resources and know-how to do so, they could build a bridge above the heights their rivers are known to rise to and limit how much land they're taking up to what they need for farming. And even then, a lot of their farmland could be replaced once they are advanced enough to use hydroponics, aeroponics, and the like to grow a lot of their food on the bridges. That leaves more land for old growth forests, wildlife, and ecosystems writ large to flourish.

The main concerns I can think of would be how the foundation of the bridges would affect the riverine ecosystems, how the bridges might change the downstream flow of rivers (which could easily affect many other aspects of the larger ecosystem in the area), and how the handling of waste products might more directly pollute the water.

There are probably many other factors that I've failed to consider, but I'd like to know what the main concerns would be and how this sort of city would measure up, sustainably speaking, to a more typical land-based city. For the sake of argument, assume that the people in this city behave similarly to current eco-conscious populations (e.g. in Scandinavian countries) and, if I may ask, please compare the results to those current land-based populations. If you wish to expand the scope, I'd greatly appreciate thoughts as to the differences for more and/or less eco-conscious populations, too.

To clarify: my question is whether two otherwise similar peoples, one living on bridges (not needing to clear out land for themselves) and the other living on land, would have any significant difference in their footprint. I fully understand that many other factors probably play much larger roles in a society's footprint, but I'm concerned with comparing a bridge-based city with a similar land-based city.

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    $\begingroup$ What is your question? $\endgroup$ – sphennings Feb 21 '18 at 1:06
  • $\begingroup$ @sphennings My question, essentially, is whether two otherwise similar peoples, one living on bridges (not needing to clear out land for themselves) and the other living on land, would have any significant difference in their footprint. $\endgroup$ – Aporia Feb 21 '18 at 1:13
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    $\begingroup$ I would really like to see some calculations and maybe a drawing explaining where will you put all the people who live in the city, their industry, and their agriculture. Rivers are tiny in terms of area, as in, really really tiny, and bridges are even tinier. Consider for example London; look at a map, compare the area occupied by London with the area occupied by the Thames. $\endgroup$ – AlexP Feb 21 '18 at 4:02
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    $\begingroup$ @F1Krazy yes, London Bridge was built on: gettyimages.co.uk/detail/news-photo/… $\endgroup$ – pjc50 Feb 21 '18 at 10:18
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    $\begingroup$ The bigger the city, the less relevant the question. Cities eventually pave over rivers (London has paved over quite a few). There is likely ony a distinction when we're talking about a hamlet. What is the population of the "city" we're talking about? (A specific number, please.) $\endgroup$ – JBH Feb 21 '18 at 13:15
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Not really, most of the polution we as human produce happens on the industrial sector. It don't matter that they are living on a bridge if they are still using heavy scale mining and production of unecessary products in order to maintain an economy based around consumerism.

If you want a more eco-friendly civilization what you need is to reduce individual consume and have a culture that encourages repair and maintenance, instead of disposable products.

First thing you need to understand is the size of cities, we are talking about millions of individuals sharing space and needing resources. There would be really few rivers in the entire world that could accomodate the number of bridges that could even start to accomodate these people and their assorted industrial requirements.

Then you have industry and agriculture, unless you have super eco-conscious people those too will still need large swathes of land to produce the requirements of your society.

(Forget Scandinavina countries and start thinking about eco-anarchist comunes)

Its hard to estimate the damage to the river simply because we never have tried any project in such a scale, you would end up covering most of the river, disturbing not only the fishes, but any other creature that feeds on them

Then you have change in temperature of the water that will completely wreck the microbiota, add to that the ejection of untrated waste directly from the bridge-city and you will certainly have algae blooms almost constantly.

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    $\begingroup$ Most part of the forests have been cut off to open space for agriculture, not habitation. $\endgroup$ – Sasha Feb 21 '18 at 1:18
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    $\begingroup$ Well, now things start getting complicated. It would depend a lot on exactly what system you would be using to produce food, how you will fertilize it, how you intend to dispose of the waste, etc. But, as a blanket statement: Yes, you would seen a decent reduction on the eco-print $\endgroup$ – Sasha Feb 21 '18 at 1:31
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    $\begingroup$ Bridges are relatively small projects that affect a small area, they sure look big, but the total area of the river they affect is minimal. For being able to support a population of millions you would need a lot of bridges. $\endgroup$ – Sasha Feb 21 '18 at 1:37
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    $\begingroup$ The problem with building skyscrapers on a bridge is that they will have no solid foundation to sustain themselves. $\endgroup$ – Sasha Feb 21 '18 at 1:40
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    $\begingroup$ The idea has its potential, but there's one thing you need to remenber: In ecology there are rarely a single right answer. You want to use what you have avaible on the local, work with what you have, and explore diverse ideas. $\endgroup$ – Sasha Feb 21 '18 at 1:43
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If you have a continuous bridge over a significant length of river, you'd be blocking off the light to it, consequently killing algae life and rendering the river sterile. (Since plants are always the base of the ecological pyramid.) That's not Eco-friendly at all.

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  • $\begingroup$ That's something I hadn't thought of. Thank you. For clarification, though, the city would be a series of bridges built from bank to bank, not down the length of the river. In your answer it seemed to me that you thought the latter when I meant the former. Otherwise, is that a current problem in cities with many bridges that cross over the rivers that run through them? $\endgroup$ – Aporia Feb 21 '18 at 1:21
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    $\begingroup$ @Aporia, Hmm. From what you described, the image I had in my head was of a very wide bridge: from bank to bank, but stretching to quite some length along the river. Currently, only a small percentage of the river surface is under bridges in any city (except for rivers that have been completely turned into subterranean canals, like River Fleet in London). Consequently, the disruption of light reaching the river isn't significant. But if a significant part of the river surface is built over, things would be different. $\endgroup$ – Galastel supports GoFundMonica Feb 21 '18 at 1:34
  • $\begingroup$ Ah, I think I see where the confusion came in, then. I'll try to clarify my question when I have more time. I meant a series of bridges that are probably slightly wider than usual, but not by any more than minimally necessary (if at all). I was thinking of a thoroughly modernized London Bridge (when it was habitable) or Ponte Vecchio (but over a wider river) that is repeated a few times over in succession on the same river, with ferrying between them. I hope that clarifies things for you. $\endgroup$ – Aporia Feb 21 '18 at 1:41
  • $\begingroup$ This is beginning to look less like a city built on a bridge and more like a city with a lot of bridges over a river (or maybe a number of rivers) $\endgroup$ – Lee Leon Feb 21 '18 at 9:34
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    $\begingroup$ Make it wide enough and eventually you don't have a bridge over a river. You have land over a sewer pipe. $\endgroup$ – candied_orange Feb 21 '18 at 10:56
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Ships

Rivers have been important to civilization since the beginning because sailing up and down the river is much easier than trudging over land.

A standard bridge will stop ships from passing! Everybody up-river is going to hate you and try to destroy your bridge/city.

You can build a drawbridge or something, but you don't want to build houses on a drawbridge.

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When it comes to the scenario you postulate there is really one problem with it, the only way to make a bridge city that can fit the population of a major city would destroy the river's ecosystem. It would just cover up to great a length of river! now that doesn't mean the idea of bridge cities is flawed, it would just be on the scale of a large town rather than a city. The bridges should also have a highway or railway running through them (preferably both) that the people would have access to.

I also would like to point you to other types of cities that would probably also exist. First of, going with the idea of living on water, the "budget" floating city. This is basically just a cruise ship, imagine a country like Tuvalu buying a cruise ship and placing it within the atoll to free up space on land for food production, also they could have food and electricity production on the ship.

There's also just digging underground and building higher up. Anything to shrink the size of cities and increase in walkability is what you're looking for. Building developments on Bridges and in tunnels are just ways to not build on land but nothing in itself will be everything you need. Watch some Ted-talks on city design to understand what they should look like.

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