Rivers around the world are under increasing pressure from a variety of human activities.
Effective management of riverine landscapes requires an ecosystem approach and one that
recognises the complex interactions between their physical, chemical and biological
components. Perceptions of pattern and process are central to our understanding of riverine
landscapes. Pattern and process operate over multiple scales to produce heterogeneous
mosaics of landscape patches that change over time. Hierarchical patch dynamics provides a
useful approach to unravel pattern and process at multiple scales in riverine landscapes. This
thesis adopts a hierarchical patch dynamics approach to investigate floodplain sediment and
nutrient dynamics within the Barwon-Darling River in South Eastern Australia.
The flow regime of the Barwon-Darling River is highly variable. As a result, it has a complex
channel cross section featuring inset-floodplain surfaces that occur at multiple elevations
within the channel trough. These surfaces formed the focus of this study. The texture of inset- floodplain
surface sediments displays a patchy spatial distribution and one that did not reflect
lateral or longitudinal gradients within this floodplain landscape. Rather a sediment textural
patch mosaic was identified. Nutrient concentrations associated with the surface sediments of
the inset-floodplains were also shown to vary significantly resulting in a nutrient patch
mosaic. This spatial nutrient mosaic was enhanced by factors including the surface elevation
of the floodplain surface.
Sediment and nutrient exchange between the river channel and inset-floodplain surfaces was
measured during several flows in 2001, 2002 and 2005. Pin and sediment trap data showed
that significant quantities of sediment were exchanged between the river channel and
floodplain surfaces during inundation with both cut and fill processes occurring. Patterns in
sediment exchange appear to be related to local sediment supply and seasonal sediment
exhaustion, rather than the top down geomorphic constraints considered. These material
exchanges resulted in a change to the spatial configuration of the sediment textural patch
mosaic. Distinct new sediment textural patches were created following inundation, while
other patches were lost post inundation and other patches changed sediment textural character
to move into pre-existing patches. Thus a truly dynamic sediment textural mosaic exists
within this floodplain landscape.
Nutrient concentrations associated with floodplain sediments also changed over time. While
nutrient concentrations increased after the December 2001 flow event, they generally
decreased after the March 2002 event, highlighting their dynamic nature over time. The
spatial distribution of nutrient concentrations also varied over time, with a 40 percent change
to the nutrient mosaic as a result of the March 2002 flow event. In addition to the influence of
the changing physical template (sediment texture mosaic), nutrient concentrations were
shown to be influenced by rainfall processes on non flooded surfaces, and also a number of
top-down constraints and bottom-up influences operating over multiple spatial scales.
Overall, the inset-floodplains studied in this thesis acted primarily as sediment and nutrient
sinks, and were a source for dissolved nutrients. Nutrient exchange was associated with the
exchange of sediments in this riverine landscape, over both inter-flow and decadal timescales.
It was demonstrated that water resource development within the catchment reduced the
number, magnitude and duration of flow events down the Barwon-Darling River and as a
result reductions in the exchange of sediment, associated and dissolved nutrients between
inset-floodplains and the main river channel were calculated. The greatest reductions were
with the release of dissolved nutrients (42-25 percent) and the exchange of sediment and
associated nutrients from high level surfaces (43 percent).
Effective conservation and management of riverine ecosystems must occur at the correct
scale. This study identified potential nutrient hotspots at several scales in the Barwon-Darling
floodplain landscape that could be targeted by management. The low predictability of the
location of nutrient hotspots at the inset-floodplain scale over time means that environmental
flows should be targeted at high level surfaces (<25 000 MLD-1) that provide long term
sources of carbon to the river channel. Conserving flows of this magnitude will also reinstate
flow variability, an important facet of the Barwon-Darling River?s hydrology that has been
changed by water resource development. The research presented in this thesis highlights the
importance of not only considering pattern and process at multiple scales, but also the way in
which these processes influence landscape patterns over time, leading to the identification of
the appropriate scales that can best be targeted for the conservation of these systems.
| Identifer | oai:union.ndltd.org:ADTP/203491 |
| Date | January 2008 |
| Creators | Southwell, Mark, n/a |
| Publisher | University of Canberra. n/a |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Mark Southwell |
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