Is a river canyon realistic here? Why or why not?

[eepjr24]

Okay, for story reasons I would like to put a river canyon at a spot on the map outlined in red below. I know I can hand wave it and just say it was magically formed, but I would really prefer to keep that sort of thing to a minimum in the beginning as I learn where terrain features can and cannot go. Thoughts from the experts?

kervria_20201126_2.jpg

- Ernie

[eepjr24]

More information:

This large island (small continent?) has the 15 S latitude running across the top, just south of the northern most peak. The 30 S runs a couple hundred miles south of the island. So that puts the snow line where I would want the canyon to come from at about 14-15,000 feet (4,250-4,575m) altitude. The highest peak in the range (south and slightly east of the canyon start) would be about 22,000 feet (6,700 meters). So think Southern Peru to Northern Chile in climate. The rainfall will be pretty high, probably greater than 200 inches per year (5,100 mm), with some freezing at the higher altitudes of the river, contributing to canyon formation through the freeze thaw cycle. The southern side of the mountains I am thinking of a rainforest in the semi-circle created by the mountains as the soil and rock are more absorbent on that side of the range.

My best guess on the canyon depth would be about 4-5,000 feet deep at the start and drop to a few hundred feet deep where it meets the ocean. The widest point will be a couple miles (~ 3km) wide and the narrowest less than a mile (~ 1,500 m).

Thoughts? How to I summon the river police to take a look at this?? =P

Seriously, anyone with more geological / climate / topographical knowledge who sees any kind of problems please feel free to let me know, I am reading what I can find on canyon formation but the stuff I understand easily is pretty basic and the technical stuff is hard to find.

- Ernie

[NeonKnight]

Yep, could totally have a canyon there.

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

the kicker is, if you want it without the handwavium, then the river would have likely formed along a fault of softer rock (limestone) between harder rock, and over time, the water erossion along with wind, would wear down the softer material (limestone/Sandstone) before the harder material (granite/basalt).

[waldronate]

Additional information is always good! It was hard to guess about plausibility without a motion of scale. As NeonKnight pointed out, it's certainly possible to get what you're asking about.
I would add that information on the intended width of the canyon would be good. Something like a slot canyon the better part of a mile deep is very hard to do. Something like the grand canyon, zion canyon, or yosemite is much easier to do. However, getting that much erosion over that much distance straight down from mountain peaks might be difficult without l larger area up to to deliver a concentrated stream of water.

[eepjr24]

Additional information is always good! It was hard to guess about plausibility without a motion of scale. As NeonKnight pointed out, it's certainly possible to get what you're asking about.
I would add that information on the intended width of the canyon would be good. Something like a slot canyon the better part of a mile deep is very hard to do. Something like the grand canyon, zion canyon, or yosemite is much easier to do. However, getting that much erosion over that much distance straight down from mountain peaks might be difficult without l larger area up to to deliver a concentrated stream of water.

I don't think a slot canyon is what I am after as those are very narrow, narrow enough to jump or even touch both sides?

From earlier post:
"My best guess on the canyon depth would be about 4-5,000 feet deep at the start and drop to a few hundred feet deep where it meets the ocean. The widest point will be a couple miles (~ 3km) wide and the narrowest less than a mile (~ 1,500 m)."

Any of those figures are changeable if they would make the resulting canyon more realistic. As to other scales, the island is about 1,000 miles top to bottom. Total length of the river canyon would be in the 75-80 mile range. I know there are MUCH shorter river canyons (Tallulah Gorge in Georgia is 2 miles long and 1,000 feet deep, 200 feet to half a mile wide).

Does that answer the scale questions more thoroughly? If not, what else would be good to add?

- E

[waldronate]

My reading comprehension seems to be lacking in my old age. I interpreted the second set of dimensions as as part of the first set, for some reason. I was trying to get a notion of how much water was moving over the area.

The dimension values that you have there are quite plausible for a river canyon and there are multiple examples along the west coast of the Andes in the area you specified. New Zealand also has some examples of a better-watered landscape than the Andes, though on a somewhat smaller scale.

The topography on the map is from a synthetic noise generator and implies that there would be that sort of erosion all over the place because the area that you want to have as your canyon doesn't have much in the way of a catchment basin. A little handwaving from a geologic perspective could raise that central mountain area into more of a plateau and have a river collect water over the whole area and drop it off the edge of the plateau to cut that canyon. It would be a little like the Brahmaputra river in the Himalayas with that hard turn and radical change in character over a fairly small distance.

What is the intended character of this area The mountains are actively rising from somewhere to be that high, but is it intended to be mostly volcanic in origin or something like a batholith that's bring pushed up from below? In the case of the Andes, stuff is being shoved under an existing chunk of hard rock (it's why there's that wide part east of the Andes).

[eepjr24]

What is the intended character of this area The mountains are actively rising from somewhere to be that high, but is it intended to be mostly volcanic in origin or something like a batholith that's bring pushed up from below? In the case of the Andes, stuff is being shoved under an existing chunk of hard rock (it's why there's that wide part east of the Andes).

Batholith is a new word for me, but in reading the description that seems very much like what I would want. There would be some minor volcanic activity in this range, steam vents with maybe a hot stream or the like on the other side of the highest peak. Some spots might be as active as Yellowstone, but that would be rare. No above ground lava, etc.

This "island" is supposed to be on top of the line where two plates meet, which I think also fits with the batholith? The fault would run south-south east to north-north west on the island. Quakes would be a reasonably frequent occurrence, especially in the area of the southern most range.

Does most of that sound reasonable? Thank you for the assistance, I really appreciate it!

- E

[waldronate]

If you look at the Sierra Nevada mountains in California, that's an example of a large batholith that's being pushed up as part of a collision with the North American continent. There are a lot of other things going on in that area and it's hard to get things like that happening in isolation. Note that Yellowstone is a special case as it is a hotspot burning through the crust (see the Snake River Plain for an example of what it did on the way to where it is now).

A large batholith like that would need to be emplaced from something like a previous continental margin. Only much later would something push it to the surface. The fun thing about that sort of behavior is that you can get nice linear features (see the Kern River Canyon in the southern Sierra Nevada mountains in California) caused by breaks in the batholith.

As far as the valley goes, is your intent to have it get narrower or wider as it heads to the sea? I will admit to being curious about its significance and whether it needs steep walls and a flat bottom or if a more traditional V shape is as useful?

[eepjr24]

If you look at the Sierra Nevada mountains in California, that's an example of a large batholith that's being pushed up as part of a collision with the North American continent. There are a lot of other things going on in that area and it's hard to get things like that happening in isolation. Note that Yellowstone is a special case as it is a hotspot burning through the crust (see the Snake River Plain for an example of what it did on the way to where it is now).

I said Yellowstone, but what I was really thinking of was more like Arenal in Costa Rica. Hot rivers that get into the 110 degree range in spots, natural bath pools, etc. This would be from volcanic activity deep inside the mountain. Not looking for active geysers, acid pools, lack of plant life, etc.

A large batholith like that would need to be emplaced from something like a previous continental margin. Only much later would something push it to the surface. The fun thing about that sort of behavior is that you can get nice linear features (see the Kern River Canyon in the southern Sierra Nevada mountains in California) caused by breaks in the batholith.

I am afraid you lost me on that. Does that mean I am better off justifying the large mountains with tectonics or volcanic activity?

As far as the valley goes, is your intent to have it get narrower or wider as it heads to the sea?

Honestly, I would love some advice on that. My initial thought was that it would be narrower but deeper near the head and widen somewhat as it slows and gets to the sea? Do these canyons sometimes have large waterfalls at their heads? That makes sense to me, but not sure if it is just wishful thinking.

I will admit to being curious about its significance and whether it needs steep walls and a flat bottom or if a more traditional V shape is as useful?

I would lean more toward the steep walls, but it will likely vary somewhat naturally depending on the material being cut through and the speed of the water at that area, correct? Basically, as long as it is very difficult to traverse terrain that makes you vulnerable to attacks from above it will serve my purposes. Magic will make things a bit more difficult, but those with the ability to move large numbers of troops in this manner will be both rare and targeted.

It will be a contested border between protectorates of two colonial powers in the area. The southern colony has been neglected since the silver and mercury mining has become more difficult (similar to Peru in the 1700's by Spain). The northern colony is more independent and somewhat similar to the US near the time of formation, but pre-Revolution. The rest of the border is either harsh mountain terrain or a very dangerous tropical rainforest on the southwest border. The conflict is more simmering than active at the moment, but that could change depending on the actions of the party(ies) who will be adventuring.

I will post an updated map to my WIP thread tomorrow at some point, my eyes are too tired to try it tonight.

- Ernie

[waldronate]

As is so often the case, the story that you want to tell can have a profound impact on the landscape, which can then turn around and affect the story. The earlier discussions gave me the impression that this was a sort of one-off, unique feature. The story information suggests that it might merely be the largest one of similar things in the area. A flat valley floor offers opportunities for hidden cities and the like, which was where I thought you might be going with the desire to have a relatively wide valley floor. A mining frontier, though, is something quite different. The craggy sides of valleys are common and landslides in wet areas are going to eat roads all of the time. Preprocessing of ores will likely cause removal of lots of trees to feed the furnaces, which will increase erosion. Local gangs of whatever's hostile will be trying to profit from a little bit of mining and from whoever is trying to go across the area.

The hardness of the material being cut by a river will have a strong impact on shape of a canyon, but the rate of uplift will also have an impact, as will glaciation. Weak materials will be cut rapidly and the valley floor will widen as the overall drop in the river lessens. Rapid uplift will give strong downcutting because the river will always have enough energy from its drop to keep heading downhill (rivers in nearly-flat areas wind from side to side and widen their valleys because they need to expend their energy somewhere). One way to get strongly-downcut area in a formerly flat area is to drop the base level (for example, lower sea level or increase land level). The river will downcut into the previously near-flat coastal plain and give you the canyon at sea level (you can also get things like entrenched meanders). Glaciation in the past along the upper reaches of the valley can give U shaped valleys with steep walls after the glaciers disappear if the rock is fairly resistant.

The presence of magic (especially advanced magic) in a mining zone is a horrifying thought. Gangs of unreinforced humans can do all manner of damage in a fairly short time. Now add in something like a lone wizard who can summon an earth elemental and you have the potential for hundreds of years of work to be compressed to a couple of weeks. Imagine that wizard getting the summoning not quite right one day and pulling in vermin the equivalent of rats that like to eat tasty and valuable ores. For the D&D minded, something like a decanter of endless water is the equivalent of the strongest monitor ever devised for alluvial mining: it never runs out of water, and it doesn't need infrastructure to do its damage. The most prized and feared thing, of course, would be the hundred meter long slug that digests rock to eat the sulfide ores and leaves a trail of oxidized metal chunks in a slimy, reeking trail behind it.