Sagan 4's current map is so mountain-dominated that there's a sharp elevation change going down rivers to their estuaries. In real life, this would create canyons. Given how long many of our rivers have existed, one could even argue that Sagan 4 has a Grand Canyon for every one of them.

If this is the case, would it be fair to treat all riparian biomes that cut through moutains as though they include cliffs on either side?

That would be TOO COOL if it was approved. As for me I fully support this idea.

I'd be fine with that, especially since it could lead to some fairly unique life living in them, especially if they're sheltered from the harsher conditions of the surrounding mountainous life and such.

I concur with the idea that several rivers have existed for long enough to carve canyons.
Should these canyons be large landmarks, for each long-lasting river, or simply narrow regular habitats where the rivers cut through mountains?

If there are not separate "canyon" and "riparian" biomes, there should be notes on the ecosystem pages saying which river systems have cliff walls.

Thinking about this again, I figure there are probably areas with and without easy ways up and down the canyon walls, as they're gonna be pretty eroded anywhere wet and they're not being treated as major barriers. They'd be too frequent in this period for landmarks to be feasible, but it could be great for diorama inspiration.