What are low-flows?
Rainfall is naturally captured in catchment areas travelling downstream in creeks, streams and rivers, supporting all living things as it goes. The natural movement of water has been disrupted by the construction of more than 8000 dams across the EMLR, disrupting the timing and volume of flow provided to the environment.
Low-flows are naturally occurring, regular, small flow events that are a vital part of the annual water flow pattern of a catchment.
These days, low flows are impeded until dams fill and spill, delaying water flows until later in the season. This change to flow patterns is a major driver of declining catchment health.
Many water-dependent ecosystems in seasonally flowing streams are in poor condition with parts of the catchments not flowing for as long or as often as they once did.
Catchment dams generally fill during medium and high flow events. Low flow devices only pass ‘low flows’, i.e. flows up to a specific ‘threshold flow rate’. During medium and large events the majority of flow goes into the dam while the small proportion below the threshold flow rate is passed. Low flows are only passed when a dam is receiving flow or when there is flow past a watercourse diversion.
Why are low-flows important?
The Flows for the Future program will ensure that there is an environmentally sustainable level of water usage and that the creeks and waterways leading to the River Murray and Lower Lakes are able to flow in accordance with the Murray-Darling Basin Plan.
Allowing natural patterns of low-flows to reach waterways throughout the year also supports the patterns and cycles of native fish, animals and plants which have evolved over thousands of years. Low-flows provide and maintain aquatic refuges, support the life cycle of water dependent plants and animals and allow survival throughout the drier summer months.
Fish communities are now struggling and in decline due to altered flow patterns that favour introduced fish species. The Southern Pygmy Perch (Nannoperca australis) and River Blackfish (Gadopsis marmoratus) are now endangered. By restoring low-flows, native fish will be able to move to spawning environments and juvenile fish will have a greater chance of surviving to adulthood, helping native fish populations to recover.
Passing low-flows (allowing low-flows to continue downstream uninhibited) will result in short and long-term environmental responses including:
- longer periods of flow
- a freshening of water pools and reduction in stagnant water
- maintenance of permanent pools
- re-colonisation of water-dependent plants
- improvement in the diversity of bugs which in turn assists in nutrient cycling
- help to control pest species such as mosquitoes
- improved stability of water courses.
Returning low-flows also helps maintain water quality in backwaters, wetlands and floodplains by flushing salt and pollutants that accumulate during dry periods.
What is a catchment?
When rain falls, the water collects in an area of land which is often bound by hills. The water flows across the landscape, into the soil and streams, eventually feeding a river.
How much water ultimately feeds streams and rivers depends on various factors including rainfall, soil moisture and topography.
Catchments are complex. When something happens in one part of the catchment, it can have a significant impact on other parts.
There are 16 surface water catchments in the EMLR Prescribed Water Resource Area .
Why do we need to pass low-flows?
Securing low-flows is one of the important policies being delivered as part of implementation of the EMLR Water Allocation Plan. EMLR water allocations have been issued on the understanding that a program to secure low-flows is implemented to improve catchment health and support primary production for years to come.
If low-flows of water are not secured then the volume of current allocations will not be sustainable into the future and catchment health will continue to decline. It may become necessary to review current allocations based on a lower volume of water availability.