Can the hydrology of this world be made to work?

[Anaraxes]

Originally Posted by Michael Thayne

What do "east" and "west" even mean independent of the planet's spin?

Not what I meant. Simply that if you spin the planet the other way around, it's the same as reversing the map east-west. The labels would also change (assuming the inhabitants find sunrise and sunset interesting), but the actual relative geography doesn't. Either the mountains are spinward of the river or they're anti-spinward.

Prevailing winds are a result of the Coriolis effect. Reverse the planet spin and you switch the direction of the Coriolis and thus prevailing wind direction relative to geographic features. Or you could flip the relative positions of the rivers and mountains (flip the map) while keeping the same rotation direction. Only if you do both reverses does the situation remain the same.

Assume the mountains start off antispinward of the rivers; other factors are such that prevailing winds come over the mountains so the rivers are in a dry region. Now, flip those mountains to spinward of the river. The planet's still spinning the same way, so the wind moves the same way, but now the wind crosses over the rivers first, then the mountains. Not dry. Now do the second flip, this time of the planet's spin. Now the mountains are antispinward of the river again, so the rain shadow is back. Either change alone will reverse your prevailing wind direction relative to the geography. Both together means no net change.

It doesn't matter to the existence of the rain shadow if the inhabitants also switch their words (which I agree they probably would, if they find sunrise as interesting as humans do), because that linguistic change doesn't flip the mountains to the other side of the river along with the planet spin or cause the wind to prevail in a different direction. The label is not the terrain.

But this point is really off in the weeds. The practical upshot is that it's merely a way you could save an existing map, assuming you had one already drawn and didn't want to redo it. (Or you scan it into Photoshop and reverse it left-to-right.) Or perhaps you want to save existing text that's chock full of references to this being "east" or "west" of that, and it'd be a chore to edit all those words to match changing planet spins.

[Michael Thayne]

Originally Posted by ericthered

You could use a double set of rain shadows: the winds blow in from the east, hit the first and lower set of mountains, then pass over a dry region before hitting a second set of higher mountains and getting completely wrung out.

As for turning the river north or south, could you tilt the river instead, giving it a slight incline into wetter latitudes?

You know, looking at a map... are the Tigris and Euphrates too crooked for you (if you mirrored them over the equator?)

Looking at a map of these rivers and their basin, Euphrates does change direction fairly dramatically somewhere around northern Syria, but that's not quite the same thing as having an L-shaped river basin, which is what I'd need here, I think.

Originally Posted by Anaraxes

Not what I meant. Simply that if you spin the planet the other way around, it's the same as reversing the map east-west. The labels would also change (assuming the inhabitants find sunrise and sunset interesting), but the actual relative geography doesn't. Either the mountains are spinward of the river or they're anti-spinward.

Prevailing winds are a result of the Coriolis effect. Reverse the planet spin and you switch the direction of the Coriolis and thus prevailing wind direction relative to geographic features. Or you could flip the relative positions of the rivers and mountains (flip the map) while keeping the same rotation direction. Only if you do both reverses does the situation remain the same.

Assume the mountains start off antispinward of the rivers; other factors are such that prevailing winds come over the mountains so the rivers are in a dry region. Now, flip those mountains to spinward of the river. The planet's still spinning the same way, so the wind moves the same way, but now the wind crosses over the rivers first, then the mountains. Not dry. Now do the second flip, this time of the planet's spin. Now the mountains are antispinward of the river again, so the rain shadow is back. Either change alone will reverse your prevailing wind direction relative to the geography. Both together means no net change.

It doesn't matter to the existence of the rain shadow if the inhabitants also switch their words (which I agree they probably would, if they find sunrise as interesting as humans do), because that linguistic change doesn't flip the mountains to the other side of the river along with the planet spin or cause the wind to prevail in a different direction. The label is not the terrain.

But this point is really off in the weeds. The practical upshot is that it's merely a way you could save an existing map, assuming you had one already drawn and didn't want to redo it. (Or you scan it into Photoshop and reverse it left-to-right.) Or perhaps you want to save existing text that's chock full of references to this being "east" or "west" of that, and it'd be a chore to edit all those words to match changing planet spins.

Ah, yeah, that's what I meant when I mentioned flipping the map in the OP, and I'd probably do the more labor intensive version if I did it at all.

Also here's very much work-in-progress map of the setting. Hexes are 100 miles. North is at the top, center of the map is 30°S latitude. White means "I am not mapping this because it's far enough away from the center that distortions from projecting a sphere onto a plane are going to get rapidly worse". Gray means "land not otherwise specified". The eastern continent is meant to account for the dryness of the river valley, while the bay in the north-west is mean to provide a way for monsoons to reach the river basin (inspired by the Bay of Bengal).

Finally, I had the thought that if the river basin has a lot of igneous rock and/or clay (edit: or limestone), it could reduce the amount of rain you need to account for the same amount of discharge downriver, because less of the rain ends up as groundwater. Not sure how far that concept really extends, though. Looking at the size and annual precipitation of the Ethiopian highlands, it looks like maybe only 5% of the rain water actually makes it to Aswan. IDK what that percentage would be if much of the river cut through less permeable terrain.

[ericthered]

Originally Posted by Michael Thayne

Looking at a map of these rivers and their basin, Euphrates does change direction fairly dramatically somewhere around northern Syria, but that's not quite the same thing as having an L-shaped river basin, which is what I'd need here, I think.

I was actually referring to the rivers not flowing strait east.

If you want crazy river shapes, there is always the Niger river, but I don't know that it will actually look right: truth can be stranger than fiction.

[Michael Thayne]

Originally Posted by ericthered

I was actually referring to the rivers not flowing strait east.

If you want crazy river shapes, there is always the Niger river, but I don't know that it will actually look right: truth can be stranger than fiction.

Oh gosh, the Niger river does offer some interesting possibilities, doesn't it. Thanks for the tip.

[Michael Thayne]

Did a bit more reading on the Niger river and apparently it was once two rivers that got joined together and it just so happened that the original two rivers flowed in directions roughly perpendicular to each other, hence the river that makes a 90° turn. That's amazing and I just may use that. Thank you so much!