BOX 2-1
Components of Flow—the “Master” Variable

Because streamflow is strongly correlated with critical physical and biological characteristics of rivers, such as water temperature, sediment transport, channel morphology and habitat diversity, it represents a “master variable” that influences the functions of associated riparian areas.

Flow magnitude represents the amount of water moving past a given location per unit time. It can influence rates of solute, suspended sediment, and bedload sediment transport, and thus is a critical variable with regard to the creation of alluvial landforms (e.g., point bars, floodplains streambanks, and channel sinuosity). As discussed later in this chapter, high flows are needed for some species to create local zones of erosion/deposition for seedling establishment.

Flow frequency refers to how often a flow of a given magnitude is equaled or exceeded over some time interval. Flow frequency, in combination with flow magnitude, indicates the amount of energy a stream has to do work (e.g., sediment transport, channel adjustments, etc.).

Flow duration represents the period of time associated with a specific flow magnitude. From the perspective of riparian plant communities and floodplain functions, flow duration represents the length of time that overbank flows occur or that soils remain saturated from high flows. Flow duration is often a crucial variable for many riparian plants that have adapted their physiology to accommodate extended periods of high moisture levels.

Flow timing generally refers to the seasonality of a given flow. For example, the timing of most snowmelt runoff for many western streams and rivers occurs in late spring and early summer. Fish and other organisms have adapted their life history strategies to the timing of these flow periods. Superimposed upon the long-term water and sediment budget of the watershed, flow timing determines the relative wetness or dryness of the adjacent riparian area and is therefore a primary structuring process.

The rate of change in streamflow or water levels represents how quickly a flow changes from one magnitude to another. Streams and rivers that derive their flow from snowmelt are generally considered less “flashy” than those that respond to large amounts of rainfall. Rate of change can influence water sediment transport rates and riparian plant communities. For example, seedlings of deciduous woody species may need a relatively low rate of change during snowmelt recession flows for them to successfully establish.

full channel, is the discharge of the 1.5- to 3-year return period storm (Dingman, 1984). Floods are of larger discharge and generally occur less frequently than bank-full events. Floods move and sort sediments and other materials, forming the physical structures that compose the riparian areas of rivers. Big floods, which are relatively rare, often create a physical template that is continually

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