Hi Katarina and Maud,
Without being able to contribute much to answering your question I think part of the problem with hysteresis is that it can be a bit cryptic. You will need quite a lot of measurments on a rating curve to be able to discern what is hysteresis and what is uncertainty or just stochastic variation due to some rocks moving here and there.
To further complicate things, I would also expect stations where there is a lot of measurements to have man made weirs to a larger extent, and therefor to be less prone to hysteresis. I might be dead wrong here though.
However, I have seen some brilliant examples of hysteresis on rating curves created by Mark Randall. In his example he used a fixed camera to capture video for surface velocimetry and did several hundred discharge measurements during the course of a year or so and the hysteresis was very evident.
/Daniel