Earlier I asked a few questions about one particular part of my setting, which is an Earth in the immediate aftermath of a supernatural apocalypse that made the world a gridded globe of 2x2 mile "squares" (pillars getting larger going up and smaller going down, along latitudinal and longitudinal lines) that are each subject to a different supernatural gimmick. The first of these "squares" that the heroes have to deal with has invisible walls stretching 100 feet (30 m) off the ground (it's in a flat stretch of a suburban town), and rain is constantly falling at the incredible rate of an inch per hour. Upon closer inspection it's later revealed that the rain isn't actually falling from clouds, but rather appearing in midair slightly above the height of the invisible walls.

This isn't infinitely generating water that will eventually cause the world to be completely flooded millennia down the road or anything; another square somewhere in the ocean is providing the water, though just the pure H2O and not the salt or other stuff mixed with it. But it will keep filling with water forever, and overflowing off the sides when it finally gets filled to the 100 foot (30 m) line roughly 50 days later.

My previous question dealt with how long it would take before that water spilling off the side would become drinkable. My new question has to do with what exactly that eventual four-directional waterfall would do to the surrounding terrain. My primary guesses are that it'll either forcibly carve out one or more new rivers, or turn everything south of the thing into mucky swampland, but I really don't have clue one, so I thought I'd ask here.

Some extra info: the setting is a fictional New Jersey suburban town called Silverwood. The 2x2 mile area of town "The Fishtank" is in is almost entirely flat, but to the south the land slopes somewhat downward, and to the north the land slopes upward. The area is largely developed, with plenty of trees but nothing resembling "wilderness" for miles around.

Once this flooded barrier starts spilling water over its invisible walls, what will that water do to the surrounding area?

  • 1
    $\begingroup$ Depends entirely on small differences in elevation in the surrounding landscape. I thing from a literary standpoint you have the license to have it do whatever you want. I'm thinking a shallow boggy lake forms draining out into a stream. $\endgroup$
    – DWKraus
    Commented Apr 13, 2021 at 21:23
  • $\begingroup$ Is the water entering 4 new squares with invisible walls? $\endgroup$
    – Hukk2010
    Commented Apr 14, 2021 at 6:51
  • $\begingroup$ @Hukk2010 No, only the original square had invisible walls. $\endgroup$ Commented Apr 14, 2021 at 10:20

2 Answers 2


Local effects

This is just to add to the other answer for completeness: You don't have an especially high flow rate, and there's a fair chance your water will largely "stick" to the edge of the region and pour down in a rather unimpressive fashion - no pretty waterfalls without some drainage works.

How fast? About 2.8L comes over each 10cm section of wall every second. (Roughly 2.4 gallons per foot, each second.) That will vary - less overspill at the corners, more in the middle - but let's treat it as a starting estimate. That'll be enough to carve out a channel over time in soft soil, and if your former residents have time, they'll probably want to do some diversions to keep it away from intact buildings.

Physics and some calculations

From your other questions, I understand that there's a way to travel between these areas, and the boundaries aren't usually so obvious. I'll assume that normal physics works through this boundary, but water molecules are 'magically' prevented if they try to leave. That leads to an interesting effect: convection. (It also trapping your hand inside if you stick it through the barrier, so you may not like the assumption.)

The waterfall pouring down the outside will pull (by normal friction) the water inside with it. Over time, you'll end up with a stable kind of turbulence, with the water on the outside of the barrier moving only slightly slower than freefall. Without air resistance, that would be about a 2.5-second drop (assuming the adjacent square has normal gravity), so let's assume the water reaches the bottom at about 20m/s. That's enough to cause some nice turbulence, so you'll end up stirring the inside of your water-pillar.


If electricity isn't being provided normally, you can run a fairly respectable hydroelectric generator off this. Pure water is also less conductive than normal water. This area might also make a good organic-waste dump, since you might be able to fish in it if you can get some nutrients in - you're most likely to get a mix of algae and carp, if some people had pet goldfish.

But the biggest use is the water itself. A ready supply of fresh, clean drinking water after an apocalypse will be a very significant resource.


Your discharge is small enough that the effects are fairly predictable. The water will cut a path to the nearest river, making a fairly large stream as it goes.

Here is an incredibly useful map showing all the waterways in NJ, so just figure out which is closest. The creation of the stream will be fairly destructive, it will cut a fairly deep gouge, effectively creating a brand new stream, undermining and destroying any structures on its path. this is destructive on the small scale but restricted to the path of the stream. NJ soil is fairly loose so it will erode quickly, and it will destroy roads and buildings in its path. But it should stabilize fairly quickly days to weeks before it is just another stream in a state full of them.

Note there will be a lot of erosion around your box from the falling water, again NJ is mostly covered in loose soil so it will erode quickly, effectively forming a deep moat like lake around the box in a couple of days.

It is also worth mentioning that since the area is flat you will not get much electrical generation out of the stream, not unless your overflow is only in a single area. For electrical generation ideally you want enough of a height difference to turn a top throw water wheel or turbine, otherwise you can only generate a relatively small amount of electricity.


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