The proposed Macquarie Re-regulating Structure would have a major negative impact on the river ecosystem, reducing biodiversity and reducing native fish populations. There are four major impacts:
- Capture of tributary flows
All environmental water is not equal. River flows that are uninterrupted by dams and weirs have extremely high ecological value, compared to flows that are stored in dams and weirs and re-released.
Uninterrupted river flows pick up nutrients (especially carbon such as dead eucalyptus leaves) and generate natural productivity of plankton, which is the essential food source of fish larvae. This is the fundamental process of river ecosystems that sustains native fish populations.
If flow is uninterrupted over long distances, it has even greater ecological value as this enables fish that are a long distance downstream to detect the increasing flow (fish can sense the slightest increase in water velocity and have an extremely acute sense of smell) and migrate upstream to spawn so their larvae have greater survival.
The advantages of uninterrupted river flow are that: it occurs with a natural season; it has a natural rise and fall in river level; and it has natural, flowing water, hydraulics. It also has no thermal pollution. All these aspects contribute to these flows having high ecological value.
In the Macquarie Valley, tributary flows and rainfall events downstream of Burrendong Dam are one of the most valuable ecological assets that are presently sustaining native fish populations. If the proposed regulator captures and re-regulates these tributary flows and main-stem flows that result from rainfall downstream of Burrendong Dam, native fish populations will have less successful breeding and populations will certainly decline.
The mitigation for this impact is to provide full transparency of tributary flows and rainfall downstream of Burrendong Dam.
- Impacts of variable water levels on river-edge and channel habitats
Tributary rivers of the northern Murray-Darling Basin have highly variable river levels, from floods to droughts. However, these water levels vary over a very consistent regime over time – rising in floods but spending a lot of time at a low level with varying baseflows. The time-scale and season of this variation is very important for fish. Nesting species such as catfish and Murray cod establish a nest in spring and if the water level drops too much and/or too quickly they abandon the nest and there is no spawning that season[1]. This is an insidious impact as it does not become apparent until many years later as old fish die out and are not replaced by young fish.
Gin Gin Weir presently has a stable water level, while the new regulator will have highly varying water levels that will vary over short times scales within an irrigation season. These are likely to impact breeding of Murray cod and catfish.
Under natural conditions in non-flood times, there are relatively stable water levels with occasional pulses of flow. These conditions enable aquatic plants to develop in rivers, which contributes to the basis for the food chain, and ultimately fish survival and ongoing populations. Regulators with highly varying water levels have weirpools that are characterised by barren banks and river channels, devoid of aquatic plants. This breakdown of the aquatic food chain results in less food for native fish, reducing their health, resilience, and survival.
- Impacts on flowing-water habitats
Rivers have a natural mix of flowing and stillwater habitats. Standing beside a healthy river, we all visually recognise flowing water and we recognise eddies, backwaters, pools and riffles – that is, healthy rivers have diverse river hydraulics (or hydrodynamics).
This hydraulic diversity provides habitat diversity and biodiversity. There are aquatic animals and plants that specifically thrive in hydraulic diversity including: natural biofilms (fungi, algae, protozoa, bacteria), diatoms, plankton, aquatic insects, snails, mussels and fish. The high biodiversity supported by hydraulic diversity supports a diverse food web, which contributes to resilience of the river ecosystem to withstand events such as droughts.
Weirs create backwater and pool-like conditions; where this happens and hydraulic diversity is reduced, biodiversity declines. That is, some species become locally extinct – they cannot survive in the semi-permanent pool-like conditions. Under natural conditions, prior to any dams or weirs, the Macquarie River could stop flowing and become a series of pools but only very rarely and for short periods of time. Notably in 1902, in possibly the worst drought on record – the Federation Drought – when the Darling stopped flowing for 11 months at Menindee, the Macquarie River remained flowing the entire time.[2]
So flowing water habitats are a foundation of the Macquarie River ecosystem.
Two key species that thrive in flowing water are Murray cod and River mussel. Murray cod are a valuable recreational fish and both species have high cultural and totemic value in aboriginal culture. Many river mussels died in the last drought in the Darling River because there were no flowing water habitats for many, many months. Although adult Murray cod can survive in large pools, where there is good water quality, the survival of larvae and young fish is dependent on flowing water habitats and the diverse food webs that these provide. Hence, to maintain the Murray cod population, flowing water and hydraulic diversity are essential to provide key nursery habitats.
The pool-like conditions that are created by weirs, not only reduce hydraulic diversity and biodiversity, but are also more favourable habitats for pest species like carp.
The proposed regulator will have three major impacts on flowing water:
- upstream of the regulator the backwater will be much greater than the present Gin Gin Weir, creating more still-water conditions and inundating Murray cod nursery habitats and River mussel habitat;
- as the level of the weirpool decreases upstream during the irrigation season, and more of the river channel is exposed, it will not have enough time to enable recolonisation of animals that specialise in this flowing-water habitat (e.g. aquatic insects, snails, mussels) – hence, critical food webs will not be established;
- the regulator will capture tributary flows and local rainfall events – therefore passing less flow downstream which will directly reduce the extent and duration of flowing water conditions.
Dr. Martin Mallen-Cooper
[1] Stuart I., Sharpe C., Stanislawski K., Parker A. and Mallen-Cooper M. (2019) From an irrigation system to an ecological asset: adding environmental flows establishes recovery of a threatened fish species. Marine and Freshwater Research 70, 1295-1306.
[2] Water Conservation and Irrigation Commission (1956) ‘Surface Water Supply of New South Wales. Stream Flow Records Period to 31st December 1950. Volume 1. Darling River Basin ‘ (V.C.N Blight, Government Printer: Sydney)
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