Kimberley Appropriate Economics Interim Report - Australian ...

3b. Vegetation change
The loss of seasonality and presence of year-round water has resulted in a considerable change
in riverine flora. Areas of sparse, temporary vegetation where plants would previously have been
washed away annually with the flood water have become dense communities of trees and bushes
growing permanently along the banks of the river.
Regulated river systems have been shown to be more prone to weed invasion than unregulated
sites elsewhere in the Kimberley (WRC 2003). The reduction in floodplain extent and inundation
frequency impacts the health of the ecosystem by restricting the dispersal of seeds and juvenile
aquatic fauna across the floodplain (Doupé & Pettit 2002, WRC 2003). An associated change in
composition and distribution of aquatic fauna would be expected.
3c. Sediment changes
Erosion and siltation within the Ord have been an increasing problem ever since pastoralism began
in the area. Despite massive dust storms and obvious erosion, negligible conservation action was
undertaken until the KDD was proposed, when it was realized that siltation would create problems
for the new lake. To combat this, the Ord River Regeneration Reserve was created in 1967 with
rehabilitation measures including the removal of feral and domestic stock, contour cultivation,
and replanting (Woinarski 1992, Start & Handasyde 2002). Despite these measures, erosion and
siltation remain significant threats to the river system.
Following the damming of the Ord, regular storm flood events (in stream) have ceased and flow
surges no longer clear the river of debris, vegetation, sand and sediment. Plants can now become
established in these areas, trapping ever more sediment, reducing the channel capacity further, and
decreasing the depth of the river. These features combine to reduce channel capacity, and produce
fewer sandbanks in the lower Ord, resulting in a loss of habitat for certain species of fish and smaller
animals (WRC 1997, Trayler et al 2002).
The average annual sediment load downstream of the KDD has changed from 24Mt pre-damming
to 0.6Mt post-damming. Of this 0.6Mt, approximately 0.015Mt comes from Lake Argyle, the rest
from the Dunham River (Rodgers et al 2000). Previously the floods would bring big sandbanks down
the river and move them around each year, now much of the sand and sediment is trapped in Lake
Argyle, with a deposition rate of 24 million m3/yr. This sediment effectively reduces the useable
storage by 600 million m3, a little over 10% of the original volume of the dam (Doupé & Pettit
2002, WRC 1998, WRC 1997).
3d. Changes in the Cambridge Gulf
Wolanski et al (2001) suggested that the East Arm of the Cambridge Gulf is now
geomorphologically unstable as a result of human activities and has changed considerably due to
reduced tidal flushing and increased sediment deposition. Suppression of large floods has resulted
in the accumulation of silt within the estuary over the past 30 years, and the width of the river has
decreased by 50%. In contrast, the West Arm (unregulated) appears to have remained more or less
stable for over a century. A further consequence of the changed hydrological regime has been a
reduction in saltwater intrusion by 50% with altered salinity of the Lower Ord (Wolanski et al 2001,
WRC 2000, WRC 1997). This is expected to have notable impacts on species and habitat diversity
within the aquatic and mangrove communities.
Riverine, estuarine and marine resources are highly dependent on water quality, environmental
flow and nursery habitat conditions, which are all critically affected by up-stream land and water
management (Volkman & Watson 2002). The implications of a changed flow regime on commercial
fish stocks and fisheries in the Joseph Bonaparte Gulf have not yet been determined. However,
studies show that changes in river flows have the potential to impact on the production of the
offshore prawn fisheries (Volkman & Watson 2002). There is substantial revenue generated from
commercial fishing offshore and fishing-based tourism in the East Kimberley. Non-commercial
values such as Indigenous subsistence and cultural and recreational fishing must also be considered.
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