For a particular river location, the phrase flow regime is used to describe the long-term pattern of seasonal variation in flow. In other words, based on streamflow data collected over multiple years, the flow regime answers questions such as:

• Does water typically flow in the river during all 12 months of the year?
• For how many days of the year does streamflow typically exceed a certain value?
• In which month of the year is the lowest streamflow value typically observed?
• In which month of the year is the highest streamflow value typically observed?

Qualitative labels can then be applied to certain river locations. For instance, river locations with a pluvial regime are rivers for which the highest streamflow values are typically observed in a single month or season associated with a rainy season. A nivo-pluvial regime is characteristic of river locations for which the highest streamflow values are typically observed in two distinct months or seasons, one associated with spring snowmelt and the other associated with a rainy fall season.