Meeting Environmental Outcomes – A Planning Framework - Water and Agriculture: Sustainability, Markets and Policies - page 195

Days 12-16: Zooplankton densities, having at first been diluted by inflowing river
water, increase to equal or exceed pre-high flow event levels. As inundation can
increase the volume of the billabong several fold, this represents a substantial increase
in micro-invertebrate biomass.
This sequence is triggered by the initial inundation and is independent of the duration of
connection to the river. Thus, the resources that are returned to the river as flow levels
recede depend on the length of time since the original inundation.
While a return to the replication of natural flow conditions is not a practical
management objective in a working river such as the Murrumbidgee River, there are
options to enhance or recreate conditions that will significantly improve the health of the
river ecosystem, including:
the supplementation of high tributary inflows with releases from storages, to create
more frequent high flow events under moderately high flow conditions
the lowering of river sill heights to again create more frequent high flow events
the use of regulators to control the flow of water in and out of billabongs and wetlands,
and
the pumping of water into billabongs and wetlands.
Clearly, not all of these options will deliver the same environmental benefits. Short
term high flow events and river pumping are unlikely to deliver nutrients back into the
river system. While this might be achieved with the use of regulators, artificially induced
inundation events not linked to appropriate cues in the river may be of reduced value. For
example, for fish species that are cued to breed by an influx of freshwater to the river,
artificial inputs of larval food without appropriate flow signals in the main channel may
be of little benefit. However, successful recruitment for a number of waterbird species is
dependent on the presence of water during breeding and chick fledging (Leslie 2001).
The presence of water is probably the main criterion here — how it is delivered and the
means of keeping it there are probably not important. The most important points in the
context of this study are that hydrological conditions need to be linked to outcomes and
that there is a preference for the flow regime that delivers that outcome at least cost.
The framework developed in this study is applied to the supplementation of high flow
events from storages along the central reaches of the Murrumbidgee River. The central
reaches of the Murrumbidgee River are located within two to three days travel time from
the two major storage dams, Burrinjuck and Blowering. Furthermore, there are significant
tributaries below the dam walls and above the wetland areas. The flow requirements to
commence inundation of billabongs within the reaches are shown in Figure B, based on
New South Wales Government Department of Natural Resources survey data. High flow
or flood events, as they relate to individual billabongs, occur over a wide range of flow
rates. However, the majority of billabongs are connected to the river system at rates
between about 22 gigalitres and 32 gigalitres a day.