The Ord River Irrigation Scheme, now in its 38th year of operation, changed the lower Ord from a seasonally dry river that carried huge floods in wet years to one with fairly constant water levels and a continuous flow. While negative impacts have occurred above the dam wall and in the Cambridge Gulf, a TS–CRC project has found the changed flow conditions have had significant benefits for the lower Ord’s riverside ecology. Management implications of these ongoing changes are, however, unclear.
Riparian systems—riverside animals and plants—are critical components of Australia’s tropical savanna landscapes. They contain a substantial proportion of the savannas’ biodiversity and are a focus for all sorts of activities that include pastoral, tourist and agricultural industries.
The TS–CRC project, led by Dr Tony Start, from WA’s Conservation and Land Management (CALM), with help from Dr Karl-Heinz Wyrwoll of the University of WA, his research students and technical officer, Tricia Handasyde, used the Ord’s riparian systems as a model to better understand savanna riparian systems generally. However, the team’s research will also be used to help manage the Ord itself, and in planning for Stage 2 of the Ord Irrigation scheme (see boxed story at end of web page .)
The Ord River irrigation scheme began in 1963 when a diversion dam was built on the Ord River at Kununurra to provide water for irrigation. The scheme came into full operation in 1973, when the second dam was completed at Lake Argyle, and the artificial lake was filled. The lake, about 62.5 km long and 45 km wide, stores wet-season flows which are released at regular intervals into the diversion dam, keeping the latter at a constant level. While some of the water is diverted to farms, much of it is released into the lower Ord. The scheme also provides hydro-electricity.
The total area of riverside vegetation downstream of the dams is now substantially larger than it was. Many of the now common and widespread vegetation species and communities were previously restricted to small ‘refuge’ patches. These new and dynamic vegetation zones now provide habitat for many species of birds, including growing numbers of the buff-sided robin, recently removed from the endangered species list. The changing nature of the river itself means that the number of other fauna species, including crocodiles, turtles and barramundi has increased.
Before the dam’s construction the river’s environment was dominated by its ‘dry-tropics’ character, and its flow was strongly seasonal. Huge floods occurred every two to three years, while in the dry season surface water was restricted to pools. During floods, water up to 50 feet deep would roar down the channel, shifting massive quantities of sediment and stripping vegetation from the riverbed. In contrast, during the dry, there would often be no flow at all. Not surprisingly, the dynamics of the river’s flow determined where vegetation was established.
Since the dam’s construction, the river has been transformed into one with a much more even temperament; it is now akin to a wet-tropics river. Water levels are now fairly constant and there are no longer periods where the river does not flow. This effect is almost the inverse that seen in most other dams, where the wall cuts off the supply of water to areas below, devastating the habitats of plants and animals that had relied on the river’s continuous flow. In the case of the lower Ord, the regular flow has created a favourable environment for aquatic and riparian vegetation to colonise, in turn allowing more sediment to be deposited in the riverbed. New vegetation is now established more or less continuously along the riverbanks and on newly formed islands. This process has not yet reached equilibrium as vegetated bars are continuing to encroach into the channel.
As a result of the regulated flow, the relationship between vegetation and the river’s processes is completely reversed. Riverside vegetation now exerts a very strong influence over where and how the river deposits sediments.
Many of the vegetation communities have expanded with the changes to the river, but some (see boxed story on the 2000 flood) have been degenerating. In particular, vegetation that exists high on the riverbank, including river gums Eucalyptus camaldulensis and coolabahs E. microtheca, require regular flooding to regenerate and are now diminishing. The lack of flooding in these zones has also lead to significant invasion by a wide range of weeds including passion vine and buffel grass. In the dry season, these weeds act to carry fire into the vegetation growing high up on the banks.
The process of change occurring on the lower Ord is by no means complete, and it is difficult to predict where it will end. At present however, a growing number of barramundi fishers and tour operators, who rely on a navigable river, are complaining that the river is becoming too shallow for safe passage in many parts. The mouth of the river is also getting shallower—the massive tides of the Cambridge Gulf continually dump muddy silt at the mouth which was previously washed out regularly by the big pre-dam floods.
Generating flood pulses is seen by some as a management option to achieve a variety of objectives including:
- maintaining navigability of the river for fishing and other uses; flushing cumbungi (Typha domingensis) and ribbon weed, aquatic weeds which have built up because of stable water levels;
- recreating conditions so riparian vegetation such as river gums can regenerate high on banks. However, weed invasion and fire regimes may now prevent seedlings establishing even if sufficient floods could be provided;
- controlling exotic weeds on riverbanks. While there is some evidence that regular flooding may remove some weed species, it may act to spread others.
It is now impossible to turn back the clock and provide floods as large and as frequent as those of old. Yet smaller floods may wash away the benefits of the current regime without achieving the management goals. “We will have to be very careful,” says Tony Start, “not to end up with a lose-lose situation for everyone.”
In 2000, Tony Start and Karl-Heinz Wyrwoll produced a series of scenarios for the Western Australian Water and Rivers Commission that examined the impact on riparian vegetation and sedimentation of various flows, either increasing or decreasing the total volume of water moving down the river—all of these would have very different impacts on vegetation and channel dynamics.
The commission is currently determining ecological water requirements of the lower Ord so that it can construct a water allocation plan. This would identify water volume to be released down the river and how this could be engineered given the water needs for the Ord Stage 2 irrigation scheme.
The project’s research findings will also form a component of the Ord Bonaparte Project (a major initiative of Land & Water Australia, the Fisheries Research & Development Corporation and CSIRO for integrated research in the region). As well, the Ord Land and Water project (a community-driven, NHT-funded project) is drawing up an issues document for managing the Ord. The curious issue for Tony Start is that a natural resource of ecological and economic value has now been created—but does the fact that this habitat was created artifically make it less worthy of protection? After all, as Tony points out, the area’s original values were also changed by human factors: stock, weeds and fire. The question now appears to be: what are the values to be conserved when considering the management of a totally altered environment?
These new values will need to be considered in the debate about the future parameters of the Ord 2 Scheme, as well as the future management of the lower Ord itself.
Lake Argyle is the largest artificial lake in Australia, its huge reservoir collecting the wet season rains and storing them for use during the dry. Water is released continuously through hydro-turbines into the diversion dam, keeping the latter at a constant level. While some of the water is diverted to farms, much of it is released into the lower Ord. Lake Argyle is about 62.5 km long and 45 km wide, its total area covering 745 km2 and containing 96 islands. It holds 10,760 million cubic metres—triple that when in flood, and its total storage capacity is 18 times that of Sydney Harbour.
Ord Stage 2, a joint project between the NT and WA governments, proposes to develop around another 43,000 ha of irrigated land, which will mean constructing new supply channels from the diversion dam (from which all water for irrigation is taken) as well as roads and other infrastructure. According to the Kimberley Development Commission the scheme will generate many new jobs and potentially add up to $500 million to the current annual income of $56 million from agricultural production on the irrigated lands.
Floods still occur yearly on the lower Ord but their size has decreased markedly. However, in 2000 there was a major flood that tested the resilience of the new vegetation. While floods of pre-dam times would have lasted days, if not weeks, floodwater in 2000 inundated vegetation in the lower Ord for many months. Water escaped slowly by the spillway, where it was about 6.5 metres deep at the flood’s height.
Most vegetation showed a remarkable resilience and all species present before the flood are now evident again. Some communities, especially aquatics and herbs were destroyed, but are now re-colonising the river from seed left on the banks by the receding water. Boabs and some figs did not do so well, with many dying even where their crowns were above water. Other trees, though badly battered, survived as long as some of the canopy was in the air. Ironically, 2001 is seeing an equally big wet but it remains to be seen before the effects can be ascertained.