May I add, trimming the grass at the canyon's edge might be an issue.
As JonD mentioned, leave the retaining wall, if you build one, below the grass line. This will make cutting the grass at the very edge somewhat simpler, as you can run the front of the mower over the edge. If you raise the retaining wall above the edge of the cut, then a string trimmer is in order. Personally, I have a difficult time with string trimmers.
Also, not to put a damper on your plans, but as Gizzy mentioned, debris likes gullies.