Environmental Flow - Flow Regime and Components

Flow Regime and Components

Flow regime influences the water quality, energy cycles, biotic interactions, and habitat of rivers. It is possible to describe flow regime in terms of five states or environmental flow components, each of which supports specific ecological functions. The health and integrity of river systems ultimately depend on these components, which may vary seasonally:

• Extreme low flows occur during drought. Extreme low flows are associated with reduced connectivity and limited species migration. During a period of natural extreme low flows, native species are likely to out-compete exotic species that have not adapted to these very low flows. Maintaining extreme low flows at their natural level can increase the abundance and survival rate of native species, improve habitat during drought, and increase vegetation.
• Low flows, sometimes called base flows, occur for the majority of the year. Low flows maintain adequate habitat, temperature, dissolved oxygen, and chemistry for aquatic organisms; drinking water for terrestrial animals; and soil moisture for plants. Stable low flows support feeding and spawning activities of fish, offering both recreational and ecological benefits.
• High flow pulses occur after periods of precipitation and are contained within the natural banks of the river. High flows generally lead to decreased water temperature and increased dissolved oxygen. These events also prevent vegetation from invading river channels and can wash out plants, delivering large amounts of sediment and organic matter downstream in the process. High flows also move and scour gravels for native and recreational fish spawning and suppress non-native fish populations, algae, and beaver dams.
• Small floods occur every two to ten years. These events enable migration to flood plains, wetlands, and other habitats that act as breeding grounds and provide resources to many species. Small floods also aid the reproduction process of native riparian plants and can decrease the density of non-native species. Increases in native waterfowl, livestock grazing, rice cultivation, and fishery production have also been linked to small floods.
• Large floods take place infrequently. They can change the path of the river, form new habitat, and move large amounts of sediment and plant matter. Large floods also disperse plant seeds and provide seedlings with prolonged access to soil moisture. Importantly, large floods inundate connected floodplains, providing safe, warm, nutrient-rich nursery areas for juvenile fish.

Each of these flow components, or events, may be quantified in terms of its:

• Magnitude: the volumetric flow rate or level; for example, 100 cubic meters per second
• Timing: the time of year during which a flow event occurs; for example, August
• Duration: how long an event lasts; for example, 3 weeks
• Frequency: how often the event occurs; for example, every 2–3 years
• Rate of change: the rate at which flows or levels increase or decrease in magnitude over time; for example, a 0.2 meter-per-day flood recession rate.

Environmental flow prescriptions, or recommendations, are often expressed in these terms.