maryjane said:... It does work - the last heavy rain went down my trench instead of across the yard. 
Great! And every rain will wash your trench deeper and wider.
Maybe when it reaches the right size, you could plant some temporary cover crop like grasses or clover to anchor it, until you get around to buying and establishing the final plants you want.
Or plant anchoring grasses along the
sides early on, so the gulley-washers only make it deeper, not wider.
