River Flow Regimes
The temporal patterns of high and low flows are referred to collectively as a river’s flow regime. The flow regime plays a key role in regulating geomorphic processes that shape river channels and floodplains, ecological processes that govern the life history of aquatic organisms, and is a major determinant of the biodiversity found in river ecosystems. There are five components that characterize the flow regime:
- Magnitude: the total amount of flow at any given time
- Frequency: how often flow exceeds or is below a given magnitude
- Duration: how long flow exceeds or is below a given magnitude
- Predictability: regularity of occurrence of different flow events
- Rate of change or flashiness: how quickly flow changes from one magnitude to another
River in regions with similar climate, geology, and topography tend to have similar flow regimes. For example, rivers draining high mountains, such as the Logan River, tend to have relatively infrequent, high magnitude, long duration, and predictable flood events that have a slow rate of change (Figure 6 on the previous page). Rivers in many tropical climates have similar flow regime characteristics as mountain rivers, due to predictable rainy and dry seasons. In contrast, rivers in arid regions are often characterized by high magnitude, short duration floods of low predictability and high flashiness (e.g., Figure 11 on the next page).
Within regions of similar climate, local factors such as soil type, soil depth, vegetation cover, and watershed size influence the natural flow regime. For example, watersheds with deep, permeable soils will be able to absorb more precipitation than watersheds with thin, impermeable soils, and will thus tend to have less flashy floods of lower magnitude and longer duration. Large rivers tend to be less flashy than small streams, which respond more quickly to individual precipitation events. Thus, natural flow regimes can be somewhat variable between nearby watersheds. Also, although general patterns in flow regime can be determined from watershed characteristics, yearly variation in precipitation patterns means that many years of flow monitoring will be required to fully characterize the flow regime of individual rivers.