Question
How do you keep mulch on a hill?

Answer
Steep hills don't hold mulch well.  It also depends on the type of mulch material.  Large rocks will stay on a steeper slope; bark mulch won't.  I've used jute matting, pinned to the slope, so as to create a rough surface. Then place bark mulch on top of that.  If there is foot traffic, dogs, or wind, all bets are off, as is the bark mulch!

Sorry I can't be more specific - The question is a bit vague. Thanks! ~M