I'd go with timber, but not in a “just cut‑and‑palm” way. Set strong wooden posts deep into the roots, then fill the gaps with stone or compacted soil. The timber lets the wall flex with the river’s movement, and the stone gives it the mass to hold back the earth. Pure natural bark and roots won’t stand up to the pressure, so mix timber and stone for a solid, repairable wall.

Timber can stand if you treat it like a living thing. Anchor the posts deep, use bracing so the wall can flex with the flow, and seal the joints against water. It’ll breathe, but the real strength comes from how you set it up, not from the wood alone.

I’ll cut the posts, set them right, and let the wood work with the roots, not against them. Simplicity wins: a solid timber wall that flexes with the river, not a fancy fence that’ll crack before the next storm. If it fails, it’s the design, not the wood.