Hydrologists have recently come up with two new qualifiers to describe river flow fluctuations over the long-term (i.e., many years). Persistent regimes are those in which day-to-day flow values do not deviate very much from the long-term average. Erratic regimes are rather those in which day-to-day flow values may deviate significantly from the long-term average. Persistent regimes are therefore mean-dominated while erratic regimes are variance-dominated. Why should we care about this difference? First, persistent regimes are less associated with fluvial floods than erratic regimes. Also, persistent and erratic regimes have different degrees of vulnerability to landcover change and climate change.