Environmental flows, also known as instream flows or instream flow needs, are a measure of the amount and quality of water flowing in a freshwater stream or river over time. This measure is based on how well the overall water flow supports and sustains a freshwater ecosystem and the life (including humans) that depends on it.

River Ecology

When studying environmental flows, 5 elements of a river or stream’s ecology need to be addressed:

  1. Hydrology: environmental flows protect all components of the natural flow regime needed to sustain river ecosystems.
  2. Biology: as flows diminish, suitable habitat for vulnerable life-stages of fish may become threatened.
  3. Water quality: adequate flows help maintain river temperatures and dissolved oxygen necessary for fish.
  4. Connectivity: healthy rivers naturally connect to their floodplains, which need to be maintained through good land use practices.
  5. Geomorphology: high flows act like a carpenter’s hammer, constructing new habitat required for continued survival of fish and wildlife.

Image 1: Five elements of a river or stream’s ecology

Five elements of a river or stream’s ecology

Importance of environmental flows

Maintaining sufficient environmental flows:

  • ensures there is a secure supply of drinking water
  • maintains healthy aquatic ecosystems
  • provides a reliable supply of water for a sustainable economy


Environmental flows help maintain a healthy fishery. For example, sufficient water flow supports the natural sediment balance of rivers and provides fish with enough water to move up and downstream for spawning.

If environmental flows are not maintained, the river can become slower, narrower and shallower. This can change the river's suitability as fish habitat, meaning that as the environmental flows change, the species of fish that can thrive there will change.

Natural flow regime

There are 5 elements to the natural flow regime:

  1. magnitude
  2. frequency
  3. duration
  4. timing
  5. rate of change

Flow in a river is naturally variable, with changes in flow within a year and changes in flow from one year to the next. All elements of the natural flow regime play a critical role in sustaining native biodiversity and overall ecosystem integrity in rivers.

All rivers have variable flow values (for example, high flows, low flows) and this variability is critical to their well-being. Variation in flow is important since it periodically restores different physical, chemical and biological functions essential to the ecosystem.

In any given river, some species do well in high flow years and other species do well in low flow years. Therefore, a single flow value (minimum, optimal or otherwise) cannot simultaneously meet the requirements for all species or maintain a fishery.

Human water use

People use or manage river flows and lake levels for a number of reasons, including:

  • flood protection
  • industrial processing
  • irrigation for agriculture
  • power generation
  • recreation
  • transportation
  • water supply for drinking

Removing water

Removing water from a river affects all 5 elements of riverine ecology (see diagram above). For example, water withdrawals can affect the water chemistry, such as temperature and dissolved oxygen.

Fish have specific tolerances for temperature and dissolved oxygen is what they need to breathe. A change in temperature or dissolved oxygen can have important consequences for fish and other aquatic organisms.

Taking too much water out of a river or lake causes stress to fish and other organisms that rely on this water. The Environmental Flows Program works at understanding what is 'too much' and interacts with others to protect these flows.


Dams can remove natural variability in river flows. For example, the high water flows that are necessary to move the sediment which maintains the shape and structure of river channels.

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