mcdonald:
My farm has a wet weather gully along much of the North boundary. It is wet 24/7 for 9 or 10 months a year, with a water level that varies from a trickle to a heavy flow depending upon precipitation.
When we built our pond in Fall 2001, I finally finished clearing out the old farm dump to put in a cattle watering station below the dam - didn't want the cows stepping on broken glass, etc. The dump was on the property line, right next to the gully which had eroded a couple of feet of elevation out from under the fence there.
To block the hole under the fence and try and prevent it's recurrence, I built a rock wall up to two feet high on the property line and filled behind it with topsoil. It was built with "native stone" (i.e. rocks of all sizes laying around the farm and dug up during pond construction), size and shape selected then fitted together by hand.
You can see from the pictures below that it has held up pretty good through over four years of gully washing. Maybe Bob or other riprap experts can point out any errors I made which could improve your effort.
The gully side:
and the cattle side:
