Environmental Engineering Reference
In-Depth Information
semi-arid and arid conditions in the southwest. The northernmost parts of the
catchment extend into sub-tropical climates that can provide sporadic high
flows in summer-autumn. The catchment was settled from the 1840s and now
provides 40% of Australia's agricultural gross domestic product; however, this
has come at considerable environmental cost. In excess of 80% of the 14
10
9
GL mean annual flow has been diverted for irrigation agriculture and domestic
use (Commonwealth of Australia
2001
). In many lowland parts of the catch-
ment, saline regional groundwater tables are perennially within the capillary
zone and large areas of the catchment have been impacted by secondary
salinisation. Nutrients are released from urban sources, irrigation return
drains and clays mobilised from exposed river banks. Stock grazing rates were
high within a few years of settlement and the controlling influence of ENSO
cycles has exacerbated erosion through extreme droughts and flood events.
The wetlands under study range from levee and oxbow lakes along the river
floodplain to terminal lakes that owe their origin to the reduction in gradient
and formation of barrier spits after the stabilisation of sea levels in the mid
Holocene. Most floodplain wetlands are shallow (
1m), and they tend to dry
regularly while some have been maintained by connection to a series of weirs
or locks along the rivers and so are deeper (1 3m). The terminal lakes are
variously influenced by tides. Of the wetlands discussed outside the MDB, Lake
Curlip is a shallow lake on the Brodribb River near the mouth of the Snowy
River catchment in eastern Victoria. Lastly, Mullins Swamp is a coastal lagoon
in southeast South Australia, hemmed in between the modern coastal foredune
and the last interglacial dune system, the Woakwine Range.
The Upper Mississippi wetlands were formed by natural barrages of alluvial
material deposited at the junctions of smaller rivers with the larger Mississippi
River channel. These represent a unique geomorphic context in that large,
linear lakes have formed in the main river channels. Accumulating material
derived from the St Croix River catchment is Lake St Croix that was effectively
dammed by deposits accumulated at its junction with the Mississippi River
(Blumentritt et al.
2009
). This catchment, of 19 900 km
2
, remains mostly
forested with some agricultural development. Average inflow in 2004 was
165m
3
s
1
(Triplett et al.
2008
). Lake St Croix is 35 km
2
in area and has a
residence time of 20 50 days. Lake Pepin, in contrast, is on the main Mississippi
River stem and was formed by sediments deposited at its junction with the
Chippewa River. Here, there is considerable catchment development for
intensive crops such as corn and soybean and the twin cities of Minneapolis
St Paul lie 80 km upstream in the Minnesota catchment. The Mississippi catch-
ment above the lake is 12 000 km
2
and the mean flow is 600m
3
s
1
. Lake Pepin
receives sediments and other material from the Mississippi and Minnesota
River catchments (Engstrom et al.
2009
). Lake Pepin is 103 km
2
and has an
average residence time of 19 days.
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