Underground hydrology question

[Tarqiup Inua]

Hello!

I have been struggling with this question for a while, now, and haven't figured out any satisfactory answer, so I figured I'd ask here.

Can you think of any (the more mundane the better) way of using waterways to travel both into and from the underground?

And how does one justify the water pouring into the underground while not eventually flooding it, as could be the case with rivers that pour into underground seas? Is there any justification provided, say, in the case of Underdark in Forgotten Realms?

I'd be grateful for any advice.

[redwizard007]

In the real world, underground rivers are more like rain fed aquifers with a flow than traditional rivers. The rain drains through permeable soil and etches grooves into the seams of sedimentary rock until it reaches an impermeable stone and then collects and/or flows along the impermeable stone, enlarging a channel as it goes. If this water breaks the surface again it is likely to do so as a spring or karst fenster, however, it is possible for a heavy flow to etch out a significant system of caverns, and powerful flows might carry enough volume to re-establish a surface river or exit into a larger body of water. There are also instances of the tunnels carved by underground rivers collapsing and thus exposing the river to the surface for a short stretch.

The only ways I have seen for surface rivers to become underground rivers is if they flow into an incredibly porous soil or are built over by man, but I am no expert on the subject.

Accessing where a collapse has exposed the river, or at a point where caves abut a sea or lake could be exploited for trade or transportation through traditional pole barges or kayaks (or barrel riding dwarves.)


The underdark in FR doesn't flood because *magic*
-Its actually because if you go deep enough in the underdark you enter the plain of shadow and the water drains away there. Technically this means Toril should be dryer than Athas, but again, *magic*

Remember, its fantasy. *magic* is actually a valid argument (if an unsatisfying one.) Most players aren't worried about why the underdark doesn't flood or how so many races exist in constant wars of genocide without destroying one another. They want to know if they can roll initiative. If you are trying to build a realistic underdark you really want to use highlands or arid areas.



For some details on real world underground rivers see the following links...

http://www.howitworksdaily.com/subterranean-rivers/

http://www.pitara.com/science-for-kids/5ws-and-h/underground-rivers/

[Mechalich]

The underdark is kind of a ridiculously open super-cavern system, with generally abrupt level changes. An underdark river is likely to plunge down steep gradients, possibly even waterfalls, as it descends from one level to the next. Without magic, it's probably navigable in one direction only.

Such an underground river could potentially emerge again, but without flowing uphill (which even in fantasy requires a lot of magic), it would probably have to do so at a much lower elevation - that's possible in certain geological conditions - ie. if you had a river that started on the Himalayan glaciers, entered the 'underdark' underneath the Himalayan plateau, and then emerged again thousands of meters lower in India.

Such a river could potentially be created if, rather than allowing it to carve canyons naturally, someone blasted holes along the route creating passage into the underdark.

And I always kind of worked with the assumption the water in the underdark eventually drained back into the ocean somehow via some really deep openings in the continental shelf - less planar cycling required that way (though probably no less geologically ridiculous).

[Grek]

For getting into/out of the Underdark via water: a sufficiently large cave, tidal forces could be a possibility. I'm visualizing a sinkhole that leads down into an underwater reservoir. A 10+ meter tidal range would not be impossible in such a situation, if the reservoir was large enough. This basically ends up being an unusual sort of lock, like they have in canals.

As for water flowing into the Underdark without flooding, it just has to come out somewhere to balance things out. Put a spring somewhere at a lower altitude and all is well. Literally, in some cases.

[frost890]

There are also gysers to get the water out along witha mountains spring.

[xBlackWolfx]

Some people who still believe that the earth is flat (yes, they still exist) believe that water does flow off the edge of the earth, but the earth is constantly moving downwards in a spiraling path, always arriving just in time to catch the water that fell off of it earlier, thus creating the rain. I kid you not.

And yeah, fantasy settings rarely make any realistic sense, its just the way they are.

edit: Oh yeah, an idea I had for a fantasy setting once, was for each 'realm' to be a flat disk hovering in space. This was inspired by the myth of Yggdrasil in Norse mythology. Rain would fall on the realm of mortals (which was located high above the others, which also explained why you can't see the afterlife realms from the world of the living, even though you can see the mortal world from the other realms), and eventually flow off the edges onto the other realms. It would continue to cascade through the realms until it got to the bottom most realm which was essentially Muspellheim, lakes of lava and everything on fire and whatnot. Of course, obviously, the water would just evaporate in this realm and drift back up to the mortal realm where it would form clouds and rain again.

[Tarqiup Inua]

Thank you for the answers, so far!

I was thinking along the lines of very large volcanic fields underground - something close to the hotter depths of the world that. The heat would cause the water spilled over them to evaporate and get out from the depths in form of steam from large steam vents... but I wonder how realistic is that. I guess it would help the air circulation in the Underdark-like places, too...

[Grek]

Less realistic than you might hope, unfortunately.

Water is really good at absorbing heat (this is one reason why people pour water onto fires to put them out) and it takes a lot of energy to convert water into steam. If the underground area was somehow hot enough to boil a whole Underdark worth of water, and had enough heat being poured into it that it hasn't already cooled down over the last few thousand years, then the whole region is basically unlivable because everything within several miles is burning hot to the touch. Steam vents and geysers only really work on a (geologically) small scale.

[MrZJunior]

If you don't object to man made objects caisson locks are interesting:

https://en.wikipedia.org/wiki/Caisson_lock

[Stellar_Magic]

Well in a volcanic environment, it may be possible to navigate in and out of an underdark environment by sailing up coves into the cave system... if the cave system is a dormant lava tube system, it probably would likely not have the sheer drop offs and other hazards that make the underdark far from navigable.

Hmm... kind of fits with the old Shetland folklore of the Drow as cave and cove dwellers on their volcanic island.

[redwizard007]

If you don't object to man made objects caisson locks are interesting:

https://en.wikipedia.org/wiki/Caisson_lock

Stranger than fiction. This goes great with a society of tinkerers.