So it makes sense that springs would form based on the theory of swales, and the computer animations of ground water I have seen. Since no one is jumping in with a response I am wondering if maybe spring formation has been overstated by some proponents of swales. I have watched geoff lawtons videos about his students property where there are amazing ponds and seemingly endless water being pumped by wind power and I love it, it seems like that person has reached a hydrological end goal. But it does not show any springs forming.
I think true springs depend on both the scale of the swale/infiltration system and the specifics of the soil and subsoil.
In our deep, deep chalk subsoil the ground water is 40m deep. Any water that gets infiltrated sinks into the permeable chalk, and I wouldn't expect to see a spring form. That said, swales could still be of benefit as the water plume in the soil is accessible for plant roots.
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There was a ravine about 8 feet deep. It was through clay. I laid a piece of 1/2" PVC drain pipe in the bottom of it. Then I built a clay dike over the lower part of the pipe. Then I filled the area above the dike with sand. Water would seep out of the pipe for about a month after a rain storm.
I built a wire check-dam across a ravine. It caught brush, slowed the water, and filled with gravel to a depth of 8 feet during the first rain storm. A layer of gravel about 3 feet deep collected on top of the bedrock for about 25 feet downstream from the check-dam. Water puddles below that where the bedrock is exposed again.
I built a berm about a foot high where some boulders stick out of a gully. Water collects on the bedrock below the boulders.
I'd say based on my experience that the potential for seep formation has been seriously under-valued... Perhaps my world view is colored by living in the deep desert. A seep out here is akin to a miracle. I'd think I was in an oasis if I had at home the amount of water that is left over in my irrigation pipe when I water a field.
So my answer for you, in line with previous replies no less, is one word: clay. I'm in northern CA and know a guy who achieved it on his hillside property. I also see things like it happening all the time in my line of work as an ecological landscaper (with some suburbaniculture on the side): where there is heavy clay (literally almost everywhere in northern CA) and an excess of water, it will pop up wherever it can downgrade, sometimes only inches below the height of percolation and sometimes a few tens of feet. It really all depends on soil texture. But it is, indeed, very possible — *in heavy soils that catch water*
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