Maras Salt Terrace, Peru
Salt would hardly have been an issue in the Okavango Delta, it's still flowing with bounteous fresh water today.
I had heard that one of the problems with irrigation canals is that salt can accumulate over time, so I thought maybe the same problem could occur with terraces. If that were the case then maybe that would explain the huge numbers of terraces all over the place in south America - they were forced to abandoned ones that had become unusable and build new ones.