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.
There are five elements to the natural flow regime:
- Magnitude
- Frequency
- Duration
- Timing
- 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 (e.g., 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.
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 from a river affects all five 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.