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DWE Home > Home > Water Management > Water for the environment > Impacts on our aquatic ecosystems

Water for the environment

Impacts on our aquatic ecosystems

Under natural conditions, river flows in NSW vary greatly from year to year and season to season. Our aquatic flora and fauna are uniquely adapted to the extremes of drought and flood. Similarly our aquifers rely on rainfall and often river flows to recharge the groundwater supplies.

The flow of many NSW river systems and the dynamics of many of our groundwater systems have been altered through water extraction and the construction of large dams. Most of our inland river systems are now controlled by one or more major dams to provide water mainly for irrigation along the rich alluvial plains, although many towns, industries and mines also depend on this water. River systems with storages that are operated to actively control flows, that is, to store water for later release for extraction downstream, are known as 'regulated' rivers. In inland NSW, these storages can regulate or change the pattern of flows for many hundreds of kilometres.

In inland NSW, the regulated river valleys include the Border Rivers (which separates NSW and Queensland), Namoi, Gwydir, Macquarie, Lachlan, Murrumbidgee, Murray (which separates NSW and Victoria) and the Lower Darling River. Along the coast, small regulating dams control the Richmond River in the north and the Brogo River in the south, with larger regulating storages in the Hunter Valley providing water for the irrigation, mining and power generation industries. Most of the water extracted in NSW is from the regulated rivers.

Major river systems in NSW (from NSW State of the Environment Report 2003)– click on the image to enlarge.

The Nepean-Hawkesbury River system has also been substantially dammed to provide water for Sydney, as have many other coastal rivers to supply water for towns and industry. These rivers, however, are not classified as regulated rivers as the water is not released downstream but generally piped from the storage to metropolitan centres. Nonetheless these storages can often substantially reduce the flow downstream. The term "unregulated river" is applied to these rivers, as well as those with no storages.

Regulation of river flow and the extraction of water for use can alter the flow regime and the natural wetting and drying patterns of aquatic areas by:

• reducing the amount and altering the timing of flood flows
• reducing the frequency of small rises in the river, called "freshes"
• prolonging low flows, thus preventing drying out of riverine habitats
• reversing the seasonal pattern of flow, with higher flows than natural in late summer (when water is being released for downstream users) and lower flooding in late winter and spring (when water is being trapped in the dam for later release).

High flows in particular are important for cycling nutrients and sediments, triggering waterbird breeding and fish spawning, for promoting vegetation growth, such as for red gums along the river, and preventing algal blooms. In addition the water that is released from large storages can be much colder than that in the river below the dam and can cause what is called cold water pollution.

The following figure shows the pattern of flows in the Lachlan River that occurred over recent years compared to that which would have occurred under natural conditions, that is, with no dams to regulate the flow and without any water being pumped from the river.

As with rivers, high levels of extraction can also affect our groundwater systems. This can result in a permanent lowering of water levels allowing surrounding saline water to intrude into the aquifer, a reduction in the base flows to nearby rivers, and a decrease in the water available to ecosystems that depend on the aquifer such as natural springs, limestone caves and surrounding vegetation. In an extreme case over-extraction could cause the aquifer to collapse.