Hill side soil erosion control around deck
You might need a professional here, because my suggestion is indeed a retaining wall. What you do in cases like this is build the wall down to a solid footing. Make sure they put rods into the hillside to both hold the wall in place and help stay the soil. The wall will better resist the water and stop the erosion. Check with local regulations, but you can certainly build walls down into a creek bed structurally.