Environmental Engineering Reference
In-Depth Information
changes. Once established, the arrival of exotic pollen biomarkers and any
switch to river plankton-dominated assemblages can be used as corroborative
evidence to support independent radiometric-based time lines.
The sediment accumulation rates in many wetlands down the Murray River
system are summarised in Gell et al.(
2006
,
2009
). The pre-industrial rates of
accumulation were typically in the order of 0.1 1.0mmyr
1
with most sites
in the lower end of this range. Pre-industrial rates in excess of 1.0mmyr
1
occurred in large meanders that were scoured deeply in the mid Holocene.
In all sites with the dating strength to allow comparison, the post-European
sedimentation rates increased, in some instances by almost two orders of mag-
nitude. Early post-European sedimentation rates were as high as 20mmyr
1
in sites near to heavily stocked sheep runs (Gell et al.
2005b
). Here they declined
post regulation owing to the trapping of coarser sediments behind weirs as
suggested by Olley and Wallbrink (
2004
). Recent rates as high as 10 30mmyr
1
are occurring, possibly driven by the increased sensitivity of floodplain surfaces
to erosion through drought and increased sodicity. Exceptionally high values
were returned from Pikes River wetland (30mmyr
1
) and Coorong South
Lagoon (18mmyr
1
). The rates returned for the estuarine Coorong sites are
considered to be net sedimentation rates as the interaction of tides can remove
settled sediments in these systems (Lawler
2005
). Along the Mississippi the
pre-industrial (c. 1830) sediment accumulation rate for Lake Pepin was reported
as ranging from as low as 0.5 kgm
2
yr
1
in the lower part of the basin to
2.1 kgm
2
yr
1
in the uppermost transect (Engstrom et al.
2009
). The post
settlement rates throughout most of the basin are typically 7 15 kgm
2
yr
1
but as high as 20 30 kgm
2
yr
1
in the upper transect. In all these values
represent a 15-fold increase relative to the pre-1830 baseline.
High sedimentation rates reflect the flux of sediment that can impact on
wetland biota through changing light regime (discussed below) and directly by
changing benthic habitat and impacting upon the function of filter feeding
organisms. Geomorphically, high sedimentation rates can directly truncate the
life of the wetland itself. About 17% of the 1830 volume of Lake Pepin is now
filled with sediment. In both the Murray and Mississippi situations, flow
regulation has reduced the prospect of flood-induced scouring and wetland
renovation. This recent net increase in sedimentation rate, thereby, represents
a clear risk to the longevity of the wetland. Given the shallow nature of many
River Murray wetlands, and the elimination of medium level floods ( Jones et al.
2002b
), the terrestrialisation of wetlands is likely to proceed at a pace greater
than wetland renewal. At pre-industrial sediment accumulation rates Pepin
Lake would have expected to have another 4000 years of life as a lake, however,
current rates allow for a projection of only 340 years until complete filling by
sediment. Many of the floodplain wetlands in the Lower Murray River system
are even shallower (
1 2m), so sedimentation rates in the order of 10mmyr
1
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