Real world geology: Greyhawk
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Davelozzi
Explorer
Yep, I'm with the Aussies here, it's certainly not impossible. I admit that I always found it unusual, but I just figured that the two outflows on the Nyr Dyv were at the same elevation, adn that the higher land in the middle that forms that Cairn Hills and Abbor Alz keeps the two drainage systems separate from each other.
I'm familiar with a similar situation on a smaller scale here in Massachusetts. There are two streams in the town of Shutesbury that flow together near a place called Pratt's Corner and then split apart and flow in completely different directions for a number of miles before both eventually join the Connecticut River in separate place in/near Amherst.
I'm familiar with a similar situation on a smaller scale here in Massachusetts. There are two streams in the town of Shutesbury that flow together near a place called Pratt's Corner and then split apart and flow in completely different directions for a number of miles before both eventually join the Connecticut River in separate place in/near Amherst.
Maldin
First Post
A branching river is not the same thing as two truly-separate outflows because, in all cases, the branches have the same slope, almost immediately rejoin, and at a exactly the same elevation. The two flows together are actually behaving as a single unit of flow as they have the exact same end point and "base level" (the elevation to which the river is trying to erode to). Any other situation is either manmade (and maintained), or VERY temporary, as the two outflows will invariably erode at different rates and the outflow with the greater rate of erosion (no matter how slight) will always capture the entire flow eventually.
So... the quick answer? Its an impossible geographic situation. Ok. Gary Gygax was not a geologist or geographer, and I'm not going to hold it against him. Can we try to explain it? You betcha!! Its magic! Ok. I'm not being fair, especially after the post I just made over in the "Poll: Fantasy world maps and real world geology" thread! But the two outflows of the Nyr Dyv certainly is an example of a situation that merits this kind of attention, so I actually did come up with an explanation using the dimensional distortions that sank the Isles of Woe. Check out the "Addendum: The Geography of a Cataclysm" section at the bottom of my The Isles of Woe webpage for a more detailed explanation that you may or may not find believable.
Denis, aka "Maldin"
Maldin's Greyhawk http://melkot.com
So... the quick answer? Its an impossible geographic situation. Ok. Gary Gygax was not a geologist or geographer, and I'm not going to hold it against him. Can we try to explain it? You betcha!! Its magic! Ok. I'm not being fair, especially after the post I just made over in the "Poll: Fantasy world maps and real world geology" thread! But the two outflows of the Nyr Dyv certainly is an example of a situation that merits this kind of attention, so I actually did come up with an explanation using the dimensional distortions that sank the Isles of Woe. Check out the "Addendum: The Geography of a Cataclysm" section at the bottom of my The Isles of Woe webpage for a more detailed explanation that you may or may not find believable.
Denis, aka "Maldin"
Maldin's Greyhawk http://melkot.com
grodog
Hero
From chatting with some geologically-inclined friends and GH members (hola Denis!), I gather that most of the northern reaches of the GH map must sit upon a large plateau, given the rivers outflowing southward into the various lakes, as well as northward into the marshes by Blackmoor, into the seas, etc. I don't worry too much about it, but I do enjoy exploring this aspect of the setting and trying to derive additional insight into what makes it tick (from a scientific POV).
Kid Charlemagne
I am the Very Model of a Modern Moderator
It might also be possible that one or the other river was linked to the lake via a manmade canal.
Whizbang Dustyboots
Gnometown Hero
Considering that the Nyr Dyv is not atop a plateau, but is instead an inland sea sitting in the middle of a plain, it still shouldn't be draining out in two different directions.Rhun said:
The cited example is not applicable.
The Nyr Dyv has to be getting more water from somewhere. Given its history, the argument that there's a gateway to the Elemental Plane of Water down near the bottom works for me.
I had to look at a map to remind myself what this situation was (negative grognard points for not having the entirety of the WoG's geography memorized 
) and I was reminded that I noticed this and tried to make sense of it once before many years ago. The explanations I came up with then were: 1) the WoG map has more extreme elevation than is readily apparent, and the north-central Flanaess is actually on something of a plateau (don't know how realistically feasible this is, but it seemed like a good enough handwave for my teenaged mind); 2) someone (likely Zagig Yragerne) must have chanelled the Selintan River and/or it must be maintained through a series of locks and canals (again, don't know how realistic this is, but it seemed good enough at the time); or 3) Nyr Dyv is literally bottomless and is being fed from the elemental plane of water. #3 was clearly the most interesting and inspirational so that's what I went with, but I've kept #1 and 2 in the back of my mind as "partial" explanations as well -- they're also true, at least to an extent. And then I never gave it another thought, until this thread reminded me of it.
) and I was reminded that I noticed this and tried to make sense of it once before many years ago. The explanations I came up with then were: 1) the WoG map has more extreme elevation than is readily apparent, and the north-central Flanaess is actually on something of a plateau (don't know how realistically feasible this is, but it seemed like a good enough handwave for my teenaged mind); 2) someone (likely Zagig Yragerne) must have chanelled the Selintan River and/or it must be maintained through a series of locks and canals (again, don't know how realistic this is, but it seemed good enough at the time); or 3) Nyr Dyv is literally bottomless and is being fed from the elemental plane of water. #3 was clearly the most interesting and inspirational so that's what I went with, but I've kept #1 and 2 in the back of my mind as "partial" explanations as well -- they're also true, at least to an extent. And then I never gave it another thought, until this thread reminded me of it.The map has bothered me too and when I used Greyhawk I came up with rationalizations;
1) The Selintan is in part a canal which may or may not contain locks, probably in the Cairn hills. This fits with Urnst and Nyrond resting in a fertile lowland along side the Nesser, a major river draining the center of the continent. Greyhawk city then is probably near the terminus of the canal.
2) One of the two rivers doesn't flow from the Nyr Dyv to the sea, but rather has its headwaters in a lowland swamp that also is headwaters for another major river. As the Selintan seems to get more traffic, I would assume the Nesser is the source of confusion.
3) The lake has filled, and continues to fill, at a rate too great for its original outlet to drain and has risen to where it is flowing out a second channel. This wouldn't likely last long.
4) The event is only recent (geologically speaking) and will not continue long. The campaign history doesn't seem to support this.
5) A vast beuracracy of water elementals herd the water of Nyr Dyv like cattle, parceling it out amongst the rivers as they deem fit.
1) The Selintan is in part a canal which may or may not contain locks, probably in the Cairn hills. This fits with Urnst and Nyrond resting in a fertile lowland along side the Nesser, a major river draining the center of the continent. Greyhawk city then is probably near the terminus of the canal.
2) One of the two rivers doesn't flow from the Nyr Dyv to the sea, but rather has its headwaters in a lowland swamp that also is headwaters for another major river. As the Selintan seems to get more traffic, I would assume the Nesser is the source of confusion.
3) The lake has filled, and continues to fill, at a rate too great for its original outlet to drain and has risen to where it is flowing out a second channel. This wouldn't likely last long.
4) The event is only recent (geologically speaking) and will not continue long. The campaign history doesn't seem to support this.
5) A vast beuracracy of water elementals herd the water of Nyr Dyv like cattle, parceling it out amongst the rivers as they deem fit.
It seems perfectly possible to me.
When I spent a summer at Yellowstone National Park, I was pretty excited about the Great Divide -- one side flows to the Pacific (via the mighty Columbia River basin), the other side to the Atlantic (via the Missouri/Mississippi basin to the Gulf of Mexico).
You'd think the Great Divide would always be at the peak of a mountain, but this is not always so -- at one point, I crossed it on a road. And stopped to take a picture of the sign. And noticed a large puddle of rainwater near the sign. With one two inch wide (5 cm) streamlet leading downhill one direction, and another flowing the other direction. If a 5' wide rainwater puddle can flow into two different ocean's, and I observed for myself that it can, than I have no problem with the Nyr Dyv's geography.
Lake Yellowstone itself has two outlets -- the Yellowstone River to the north and the Shoshone River to the east. These meet about 120 miles away, but still, it's a good example that lakes can and do have multiple outlets. If only there was a mountain somewhere along there to turn the Yellowstone River from north to NW instead of NE, Lake Yellowstone would drain into both oceans.
And of course, as Utahans know, not every water course ever does reach the ocean.
As for the claims that the Nyr Dyv must be unnatural/magical/connected to the Elemental Plane of Water, that's not necessary to explain its existence. It has two outlets, but it has 3 HUGE inlets:
-- The Velverdyva/Fals/Att basin. This massive river basin drains most of Dramidj's rainfall, from the gap of Bissel at the Bramblewood Forest north up the Yatil Mountains and all the way up to the Land of Black Ice -- for Lake Quag's outlet is the Velverdyva. Basically, something close to half the rain in the western side of the Flanaess flows through Dyvers into the Nyr Dyv.
-- The Veng/Ritensa/Crystal basin. This river basin drains Whyestil Lake (itself fed by 5 rivers draining Iuz's realm and the Cold Marshes).
-- The Artonsamay/Cold/Zumker basin, which edges up the western side of the Rakers and drains the Troll Fens and the Fellreev Forest.
A WHOLE LOT of water is flowing in there naturally, from the Yatils to the Rakers.
When I spent a summer at Yellowstone National Park, I was pretty excited about the Great Divide -- one side flows to the Pacific (via the mighty Columbia River basin), the other side to the Atlantic (via the Missouri/Mississippi basin to the Gulf of Mexico).
You'd think the Great Divide would always be at the peak of a mountain, but this is not always so -- at one point, I crossed it on a road. And stopped to take a picture of the sign. And noticed a large puddle of rainwater near the sign. With one two inch wide (5 cm) streamlet leading downhill one direction, and another flowing the other direction. If a 5' wide rainwater puddle can flow into two different ocean's, and I observed for myself that it can, than I have no problem with the Nyr Dyv's geography.
Lake Yellowstone itself has two outlets -- the Yellowstone River to the north and the Shoshone River to the east. These meet about 120 miles away, but still, it's a good example that lakes can and do have multiple outlets. If only there was a mountain somewhere along there to turn the Yellowstone River from north to NW instead of NE, Lake Yellowstone would drain into both oceans.
And of course, as Utahans know, not every water course ever does reach the ocean.
As for the claims that the Nyr Dyv must be unnatural/magical/connected to the Elemental Plane of Water, that's not necessary to explain its existence. It has two outlets, but it has 3 HUGE inlets:
-- The Velverdyva/Fals/Att basin. This massive river basin drains most of Dramidj's rainfall, from the gap of Bissel at the Bramblewood Forest north up the Yatil Mountains and all the way up to the Land of Black Ice -- for Lake Quag's outlet is the Velverdyva. Basically, something close to half the rain in the western side of the Flanaess flows through Dyvers into the Nyr Dyv.
-- The Veng/Ritensa/Crystal basin. This river basin drains Whyestil Lake (itself fed by 5 rivers draining Iuz's realm and the Cold Marshes).
-- The Artonsamay/Cold/Zumker basin, which edges up the western side of the Rakers and drains the Troll Fens and the Fellreev Forest.
A WHOLE LOT of water is flowing in there naturally, from the Yatils to the Rakers.
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