I like the chinampas idea. How large are you looking for these islands to be?
I wonder if two specific ideas would help.
One, what about incorporating something like
straw bales for both structural integrity and weight? I was thinking that you might seal a layer, or the perimeter sections, of the base of a raft made of straw bales. My thinking was EPDM, but you don't like plastics. I do think, though, that in the context of a multi-season
project like this, the use of
pond liner could be justified. It would probably extend the life of the island, and the liner might survive to see a second use.
In any case, even if they aren't used for flotation, they could easily stand in for many years accumulation of dead reeds, and serve as structure for many individual plantings that you start in the straw bales, using straw bale
gardening methods. You could prepare the whole top of the straw bale layer to accept plants, and then seed it with everything you want for duck habitat and forage, and still be able to have room and structure to accept more elaborate
garden bed plantings that you could section off from your ducks, either perennials for their
feed, or yours.
The second idea is to incorporate wicking layers, at least one on the bottom and one under the mulch layer, to maintain and regulate moisture throughout the bed.
Either way, I like the floating softwood and/or reed raft idea. I don't really like the foamed concrete idea, although I agree it would work. I just hear the idea about floating foamed concrete planters and I think about water working its way through poured concrete foundations designed to withstand that action with weeping channels because water infiltration and saturation is otherwise unavoidable. I don't like the idea of the air bubbles in the concrete filling with water over time, which might in itself cause such a structure to sink. I also don't like the idea of one of these saturated structures not sinking and continuing to pass water through the concrete into the garden bed, carrying with it whatever is in the concrete.
I also wonder why you need them to float. I understand the need for access, but there could be advantages to fixed islands built on pilings or some sunken wooden structure upon which you would then build your
land base (for which I would again suggest the straw bales, just to simulate built up dead reeds).
Lets say your slough was a long trough. If you had arms of anchored island jutting out perpendicularly to the direction of flow, alternating sides, you could lengthen the path the water travels by making it zig zag. This could be really good if you're interested in trapping sediment, making use of the islands to plant more reeds and other filtering species,
shelter and habitat for wildlife, both aquatic and otherwise, and probably a bunch of other benefits having to do with increasing edge habitats and cleaning water.
However you decide to proceed, I think it's awesome that you're doing this. If you don't mind providing more information, advice could be better tailored to your specific situation. Updates are also appreciated, because I love seeing these kinds of plans in action.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
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