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I'm not a fan of gabions. The "cages" that hold the rock together can get worn away and start to jut out into the river--creating spear points basically for fish.
I would do 3:1 slopes if you can, or even 2:1 slopes. I would install erosion control blanket on the banks and have a rock toe or even a bio-log to help hold the blanket thats in the water in place. I would sprig the banks with willow sprigs and plant riverine plants. I'd make the trench curvy and you could add rock weirs to help stabilize the grade. I would also add fish habitat structures along the banks--rootwads, jutting logs, etc. if there is fish in the river that you are building the trench off of. You could add some hard points along the trench with boulders if the trench nears a monument or whatever...but I would definately use "soft" engineering techniques.
Sounds like a neat project.
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