Environmental Engineering Reference
In-Depth Information
task if evidence for change is weak or absent. It was under this paradigm that
a catchment manager in northwestern Victoria was able to claim that Psyche
Bend Lagoon, a shallow floodplain lake lying alongside the River Murray, was
in its natural condition after irrigation waters were diverted away from the
lagoon to arrest salt fluxes to the main river. Long-term monitoring data (Gell
et al.
2002
), however, revealed that the wetland salinity was steady at around
2.0ms cm
1
for most of 1995 1997, but rapidly salinised to 60ms cm
1
within
18 months of the diversion. Clearly, facing liability for the impact on the
wetland, the manager chose to misrepresent the wetland's natural condition.
The lack of a clearly identified baseline condition of the river system more
broadly also leads to a weakening of the government's commitment to restore
it with environmental flows. A scientific reference group identified that
1500 GL a
1
of flow was required for the River Murray to have any likelihood
of achieving good ecological condition ( Jones et al.
2002a
). However, Benson
et al.(
2003
) were able to successfully claim that the degraded state of the river
had been exaggerated by the scientists and that it would not benefit greatly
from the widespread allocation of increased flow. Clearly, where issues of
liability or livelihoods are at the core of an issue, the evidence for a baseline
condition needs to be objective and defensible.
Wetlands and sediment records
Wetlands act as sediment sinks in catchments and so can be used as a means to
establish mean sedimentation rates and the flux and accumulation of pollu-
tants at the end of the system. They represent, therefore, a classic receptor, at
a catchment scale, in the sense of the source-pathway-receptor principal of
the ecological risk assessment framework of the UK Environment Agency (see
Chapter 9
). Care should be taken, however, as they have the capacity to also
act as a source (e.g., Farmer
1991
). The sediment input to lowland wetlands is
potentially greater than their upland counterparts owing to the amplification
of fluxes into higher order streams. The contribution of sediments carried by
the main river to floodplain wetlands is influenced by the degree of connecti-
vity between the river and the wetland and the frequency of floods of heights
sufficient to connect the two. These allochthonous (external) sources combine
with endogenic (internal) production of sediments and organic matter that can
further accelerate gross sediment accretion. The trajectory towards complete
infilling may be balanced by declining sediment trapping efficiency as a wet-
land fills and sediment scouring occurs with high flow events. Increased flux of
allochthonous sources down large rivers may come from surface erosion from
the catchment, river bank collapse and soil surface sodicity and this may, at
least in part, be balanced, in regulated systems, by sediment trapping behind
impoundments (Olley & Wallbrink
2004
). Endogenic drivers that accelerate
accretion rates include wave-driven erosion of exposed littoral zones and
