This gully is a real scar on the landscape in what is otherwise a beautiful part of the property. With the valley incised and the protective layer of vegetation and topsoil long gone, the sodic subsoil continues to literally melt away on the edges, just from the hillside runoff.
How to stop this?
We looked at the option of flooding the eroded section, but that would have taken a big dam wall. During a test dig, the scrape of the excavator bucket on mudstone not far below the surface showed us that most of the material we would need was well down the hill by now.