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Basically, in my world, there is a continent bigger than the whole of Eurasia, it is extremely flat (a 50 meter hill would be considered an important landmark), and its whole surface is a flood plain. I don't know if that's even possible, but I would like to know how it could be made possible, whether with a single giant river or a metric ton of smaller rivers all across the continent (if impossible, I would like to know how big could a floodplain be. The flooding should be either yearly or every other year, and it must be predictable. The flooding, if possible, shouldn't be too extreme, but should elevate enough to flood the vast majority of the continent.

Thank you for your time and attention.

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    $\begingroup$ I think one Earth example would be the Amazon river. But a large part of why it has so much rain to flood it are the Andes mountains to the west. It has many rivers that feed into a single large river. $\endgroup$
    – Brianorca
    Commented Jan 5, 2021 at 17:42
  • $\begingroup$ Huge continent would need a huge amount of water to be completely covered. That water needs to be locked somewhere and transferred to the continent, all at once. $\endgroup$
    – Alexander
    Commented Jan 5, 2021 at 18:04
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    $\begingroup$ I have taken the liberty of deleting the big questions about civilization and agriculture. These are legit questions in and of themselves and including them here makes the question too broad. The remaining question is fine (and very cool) on its own. You could link this question if you choose to ask subsequent questions about civilization on your flood world. Welcome to WB stack Rhomaioi! Off to a good start! $\endgroup$
    – Willk
    Commented Jan 5, 2021 at 19:24
  • $\begingroup$ As the first answer below indicates your continent could simply be one vast (or a series of interconnected) tidal flats that flood on a regular timetable in accordance with your desired schedule/orbital mechanics. Your BIG PROBLEM? - your insistence that the flooding must be completely predictable. In order for that to be possible you have to forgo one other important factor - weather. Assuming you are looking for an Earth like environment as your setting significant parts of your continent would also be prone to storm surges. For an example of what I mean look at coastal areas of Bangladesh. $\endgroup$
    – Mon
    Commented Jan 6, 2021 at 0:42

2 Answers 2

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Tides.

tides

tide schematic

https://scijinks.gov/tides/

High tide is flood phase for your continent. Low tide is dry land. On Earth, tides are caused by the moon. You want tides once a year and I could imagine this might work if your world were a Ganymede-like moon of a gas giant. The giant causes tides on your moon which rotates slowly. The year is caused by progress of your moon world and its parent planet around the sun. The tides (and floods) are caused by the orientation of your slowly turning planet surface as it relates to the gas giant.

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  • $\begingroup$ Ganymede is tidally locked, and has an orbital period of about 7 days. The tides would be on an approximate 7 day cycle. I think you'd want a moon that is not much like Ganymede. $\endgroup$
    – cowlinator
    Commented Jan 5, 2021 at 23:35
  • $\begingroup$ @cowlinator - tidally locked means always high tide on the Jupiter side as is the case for the oceans on Europa. A not quite tidally locked moon that still rotates a little would produce long high and long low tides regardless of orbital period. $\endgroup$
    – Willk
    Commented Jan 6, 2021 at 0:25
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You could only have a tidal floodplain; a river floodplain can never cover more than a small minority of the continent.

The area of Eurasia, combined, is roughly 55 million km^2. Whereas the largest single floodplain on Earth, the Pantanal, is only around 200,000 km^2. So it would take over 200 rivers the size of the Amazon (each) to flood the whole continent.

Except ... even that wouldn't work. The Amazon's drainage basin is over 7 million km^2. Even if we assume that the total area flooded is triple the size of the Pantanal, we're still looking at only 10% of the total drainage basin area being floodplain. Figures for the Nile and Mississippi are similar. This is inevitable because the water has to come from somewhere, and floodplains happen because of water draining from somewhere else.

So, even if your entire continent were covered in meandering river basins, you still couldn't reasonably have more than around 6 million km^2 as floodplain ... the rest might be soggy, but it wouldn't be flooded.

So making the entire continent river floodplain just isn't possible. Tides may be your only choice.

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  • $\begingroup$ So if the continent was around the size of India or even Australia (5 million square km), it could theoretically be river flooded? if not, where can investigate more about tidal flood plains? $\endgroup$
    – Rhomaioi
    Commented Jan 6, 2021 at 13:20
  • $\begingroup$ Er, no, because no more than about 10% can be flood plain, regardless of size. As for tides, see the other answer. $\endgroup$
    – FuzzyChef
    Commented Jan 7, 2021 at 7:30

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