What would be important is to mirrow the scenes when filming as the the North is in the South ;-)While I am still leaning towards the Iceland and Colorado locations, it occurred to me that there is some pretty spectacular landscape in Chile worth checking out. Patagonia is far enough south to have the correct type of vegetation and climate for what we are looking for, I think. And the advantage here is that there are a lot of river splits, creating islands that can give the type of topography we're looking for. I want to find a spot that's not too mountainous, to get the right balance, but as long as there is a relatively stable looking plateau above, I think it might work.
Torres Del Paine National Park
Parque Nacional Yendegaia
I don't know if there is a time of year when the glacial melt makes these rivers more impressive, but if so, this would be a rather interesting spot!
Conguillio National Park
This river might be an okay stand in for the Ascar, but it's definitely no Gelion. There are larger waterways in this park, though.
There's likely something suitable in Parque Nacional Isla Magdalena, as well, but I haven't found any good photos yet.
Here is an aerial video of the confluence of the Baker and Neff Rivers in Patagonia:
The Baker River is blue, while the Neff river is grey, so it is easy to see the waters mingling nicely. What I had not considered before was that it is possible the Ascar has rapids/waterfall descending to the level of the Gelion. It would make sense.
Here is a photo taken from the vantage point of the people seen in that drone footage:
And avoid filming that of the vegetation and fauna (e.g. a mara) in the background which would be obviously regnisable a not Northern hemisphere. Otherwise fine.