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.
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Originally Posted by waldronate
