# Is it possible for a moon to have a river flowing across one giant continent?

I’m creating a world where one giant supercontinent has a river flowing across it. The land is covered in lush forests and is surrounded by mountains. The world is orbiting a gaseous planet and is tidally locked. Its movement around the planet determines night and day. The river flows into the ocean as there are no mountains near the coast of the continent. Where would the river start?

• ??? Amazon, Nile, Congo, Irtysh–Ob, Missouri–Mississippi... Not to mention that in Europe we have the Rhine and the Danube, one flowing west and the other flowing east, which come sufficiently close that we have a canal connecting them so that there is a continuous waterway straight across the continent from the North Sea to the Black Sea. Dec 22, 2023 at 18:20
• rivers start in mountains flow downhill and basically never cross mountain ranges, so this all depends on where your mountains are.
– John
Dec 23, 2023 at 13:29
• @John: Rivers do cross mountain ranges occasionally. For example, the Danube cuts through the Carpathians twice, once through the Devín Gate and the second time through the Iron Gates. The Yarlung Tsangpo / Brahmaputra cuts through the Himalayas north to south through the Tsangpo Grand Canyon, the deepest canyon in the world. The Columbia Rivercuts west-to-east through the Cascades mountains. Dec 23, 2023 at 20:11
• I'm having some trouble with "The land is ... surrounded by mountains" and "...there are no mountains near the coast of the continent." Do you want to shape your topography around this river, or design the river around your desired topography? Or maybe you want to come up with some non-normal fluid dynamics? (Check out "Myst: The Book of Ateus" for an example of this.) Jan 23 at 5:30

No. Continents are higher than the oceans. Rivers flow downhill, and one river can't flow both ways, so one river can't span a whole continent.

However, you can have a river system which spans a whole continent if you put a big elevated inland lake in the middle of it (itself fed by other rivers). Have two rivers flow out from the lake system and end on opposite coasts; have many other rivers flow in to the lake system from more mountainous regions.

You can also have one river - originating from tributary streams at high elevation - which splits into two (or more) other rivers which turn so as to travel in different directions.

• You can have a river span an entire continent IF it forks, pacific creek starts in Wyoming and flows both to the Atlantic and Pacific oceans, because it splits at the Parting of the Waters one part going west to the Colombia and one east to the Mississippi.
– John
Jan 22 at 23:58
– g s
Jan 23 at 1:57

The first thing your moon would need is tectonic plates. That's pretty much the only way you're going to get the kind of crustal uplift you'd need to make this situation happen.

If you take a small, fast-moving plate and smash it into a larger plate, you'll get high mountains. I'll use Tibet as my example although there are numerous other examples on our planet. Tibet is a car-crash of an uplift, where the craton that India sits on smashed into Asia's cratons, pushing the entire region so high into the air that we're afraid to fly planes over it.

There are many rivers that radiate from the Tibet plateau like the threads of a spider web. You've probably heard of the Yangtze, Yellow and Mekong. This brings a secondary consideration, where you wonder how this can generate just one big river. For that, you need a long, linear fault like you find with the Rocky Mountains. That will produce numerous small tributaries that pour into a single river that just gets huge over the distance. The Rockies are less dramatic than Tibet, but it gets the job done.

Overall, this doesn't require much scientific justification. We really don't understand how the magma plumes inside our planet decide how they're going to push the cratons around. Nobody could tell you why it squished all of the cratons into Pangea 335 million years ago, or what happened 175 million years ago to form the Atlantic Ocean.

If you really, really want a pseudoscience justification, you could take Io as an example. Io is constantly squished by tidal forces, making its surface a volcanic hell. You don't need to go to that extreme, but a near passing of a planetoid in the moon's ancient history might have created ripples that sent a tiny, dense craton careening into a large, lush craton.

• eh, Mars has mountains, and traces of big rivers, sans evidence of plates Dec 24, 2023 at 21:54
• @AntonSherwood, I wasn't just describing what it takes to make a big river. I was shooting for the biggest river I could imagine. Mars has mountains, but no mountain ranges. That would be evidence of plates. Dec 25, 2023 at 5:52

While @g s has a good answer, here would be my solution: The river source must, as previously observed and as a matter of basic fluid physics, be appreciably higher than its mouth where it flows into the ocean. Again, as previously pointed out, this can result in it being essentially two rivers, one flowing in each direction. The height difference does not need to be severe; on a moon smaller than the earth, a difference of a thousand feet would suffice.

For desired river to be a sustainable topography feature without active tidal forces, you need some cause or mechanism that keeps water collecting at and flowing from the source; a geographic feature, or maybe some kind of magnetic or gravitational anomaly or interaction between the moon and the planet, that attracts regular rainfall, so the river(s) is(/are) a constant feature of the landscape and not seasonal. You could have an artesian spring that feeds the river constantly. (If you need a scientific cause for it, orbiting the planet can cause tidal compression that pulls subterranean water into a natural reservoir, then (gently) squeezes it up through the spring. This can be invisible to the inhabitants, as the source flow can vary very little while this mechanic operates in the subterrain.)

Or, if you want a justification for having lush forest and/or watery marsh everywhere, you can have two rivers flow INWARD from the ocean through some filtrative estuary. Such a filtrative feature is necessary not necessarily to remove salt & minerals, but to control the waterflow with a geographic feature that provides flow resistance against the columnar pressure of the seawater, which owing to the volume of water pushing against the sea mouth, is much greater than the pressure of a river would be flowing out through the same mouth. Again, this is simple fluid physics; larger volume of water, more pressure.

In the end, how the river is situated and suatained depends on your ultimate purpose in posing this question: What do you want such a river for? It's not my business to ask such a question, if only because I'm not the one who needs it answered; you, the 'sub-creator', are.