This one's simple; I am creating a wilderness map for a TT one-shot and the details of the map see a swampy bog in on portion of a valley running between the mountains, and a canyon going through the center. I keep wondering why the swamp doesn't drain into the canyon. Obviously I can make up some excuse but I don't much want to build this map on contrivances. There's already a lot of suspension of disbelief going on with portions of this map and I want to cut some of the fat off. I've researched high-altitude wetlands, but they don't seem to satisfy my cravings.
In North America, there's a species called a "beaver" -- which, as part of its lifestyle, dams streams with chaotic piles of limbs and brush coated with mud, and then builds lodges with underwater entrances in the same construction style in the ponds formed behind the dams.
These ponds, over time, will silt in and change to marshy land, the silt loosely anchored by water plants like bulrushes and similar.
When the water in the pond gets too shallow, the beavers build another dam, either above or below the old one (usually above) -- and over a period of decades a colony will transform a swift mountain stream into a winding "threaded" stream passing through a series of progressively less marshy meadows.
To have your marsh(es) alongside a canyon, the beavers would need to build their dam near a waterfall that drops into the canyon; then over some time, make additional dams upstream of the first; you'll then have a marsh just above a waterfall which is formed by the spillage over the dam (which keeps the marshy moisture from draining completely). Two such dams across the canyon from each other would give what you describe.