We've got ground water everywhere on our slopes, and for the most part it does flow year-round. However a huge portion of it creates swampy/boggy areas down the slope, just sort of resting on the surface and trickling down through the sediment. In some patches it forms a proper small stream, before dispersing again into mosquito-infested mush.
This micro-ecosystem is inhabited mostly by insects, horsetail, willow, arnica, false hellebore (trying to eradicate that one though because of the livestock), baneberry, thimbleberry, and a few other wet-loving plants.
We turned the pigs loose on the areas to see what they'd make of it. They've made some good size wallows and mostly haven't touched the actual 'creek' spots. They've mushed and mashed and reshaped a lot of the boggy spots, so they're still boggy spots just with tiny ponds in them now.
I keep thinking about these areas and in what ways we could shape them to encourage more provision and habitat for the wildlife. We have 1 pond made on the property and it's flourishing wonderfully with amphibians and life. I envisioned maybe mucking out terraced ponds down the hillside. Or just trenching a proper creek. Among other things.
I'm not against bogs. My main complaints are 1: the mosquito habitat, and 2: we want the animals to have fresh drinking water through the dry spell of summer, and the goats don't fancy murky bog water.
My main desires are to foster a source of good clean water that endures most or all of the year, reduces mosquito swarm habitat, encourages wildlife to flourish in the area, and can support some abundantly growing foodstuffs for people, pollinators, and animals alike.
Right now we have 1 slope whose water extends maybe 300 ft downhill, fairly narrow, with the boggy areas spreading no more than 30 feet wide. This slope is boggy in some spots, then turns into a proper creek in other spots as it flows. This gets water from from multiple springs, some feeding into it farther downhill.
The other slop we're working with has the same length, or more, going downhill, with its boggy areas spreading as much as 50-75' wide. Pretty much the entire trips down the hillside is swampy/mushy ground, with not clear-cut creek.
I'm curious to see what suggestions you guys might have for the areas! We're in zone 3, the bogs are in forested draws, but we're thinning the forest to 20-50 feet between healthy trees to open it up a little bit. Most of our substrate is rotten granite gravel. There is a little bit of clay under that about 1-2' down. Otherwise it usually drains really well. The boggy areas are more of a sediment-y black-ish mush zone.
There is also some info out there about using brush to slow down and encourage infiltration of water, but it seems to me that your issue is that there's too much water in the ground already and it keeps finding ways to come to the surface. If you feel it's safe to form a series of ponds (you don't want the whole side of the hill to come sliding down due to weight, and I don't know enough to give any guidance other than to suggest we get more knowledgeable permies involved!), putting a version of chinampa rafts to grow in them would be the ultimate self-watering garden! If the water is too low in organic material to feed the floating rafts, fish might solve that problem if the fish predators aren't too numerous.
It sounds as if the water would move too fast for duckweed to grow. I grow it to feed my ducks. I call it "Quack Cocai-e" (missing "n" to avoid trolls) because my Noisy Ducklings go crazy over it. Actually, my Muscovy would eat it also if I gave them a chance, but I figure the Noisy Ducklings need my limited supply more as they aren't grass eaters.
Hopefully some others will chime in with suggestions - good luck with whatever approach you choose.
I have no idea if cattails would establish here, but I love them as a plant and food source! Our pond has so far only managed to grow willow, elderberry, and a small poplar on the bank. It has some curious volunteer greenery growing in the water, I don't know what it is. Possible algae or a lichen of sorts. We introduced mint seeds and cuttings this year and have clover, strawberry, fireweed, avens, selfheal, and fireweed creeping up the embankment. I'd like to introduce cattails, but this first pond is quite small, maybe only 150-200sq ft.
The water that's on the surface isn't really flowing. It's... seeping, downward, and comes to the surface in some areas. Anywhere it manages to concentrate into one little 8-10" wide stream it starts flowing properly.
My area has some type of cattail that grows in the ditches despite it being a drought all summer, so not all varieties need an actual pond. I'm hoping to transplant a few of them in the fall to areas on my farm where their cleaning ability and biomass would be appreciated. I'm not sure I've got enough sun for them, but the only way to find out is to give them a try. This is why I want to transplant from a specific spot that is also fairly shady - I'm hoping the plants have adapted to my sort of ecosystem. All I can suggest to you is to keep an eye around your area and see if you see any that are adapted to your ecosystem.