PALEOFLOOD GEOMORPHOLOGY IN CENTRAL AUSTRALIA
Paleoflood Geomorphology in central Australia
Mary C. Bourke, Planetary Science Institute
Funding: The Honda Corporation (through the Smithsonian Institution)
The Department of Lands Planning and the Environment of the Northern Territory Government, Australia.
National Greenhouse Advisory Committee, Australia
Flooding from storms, hurricanes and cyclones is a natural but sometimes catastrophic environmental hazard. Climatic models predict that enhanced greenhouse gas levels will cause an increase in the size and frequency of floods in many regions around the world. However, the climate data we use in predictive models are mostly derived from 50-to 100-year long instrumental records (precipitation and river flow gauges). These records are too short to include the infrequently occurring extreme events that cause such devastation. Geomorphologists have found that deserts are ideal locations for the preservation of evidence of catastrophic floods. This is partly due to the fact that the rivers flow infrequently, vegetation is sparse and human impact is low. By applying geological techniques to ancient flood deposits in deserts we can get a proxy record of extreme rainfalls. This will enable a better understanding of the nature of past climate variability, the timing of extreme floods and their effect on the landscape. These data can improve our ability to predict the way in which climate may be changing and allow us to develop models on how desert environments respond to and store information on these events.
Figure 1: The longitudinal dunes system of the Simpson Desert.
The dry bed of the Todd River lies between the dune ridges and is evident by the relatively
dense population of gum trees (Eucalyptus camaldulensis and Eucalyptus microtheca).
The Simpson Desert in central Australia is composed of a series of longitudinal dunes formed by wind, that were last active approximately 20,000 years ago (Figure 1). Along the desert’s north western boundary, the MacDonnell Ranges intercept rain-bearing cyclonic depressions that cross the continent from the north and north west during the southern hemisphere summer monsoon (October-March). The largest of the resultant floods travel over 150 km to reach the Simpson Desert. It is in this location, where the river meets the desert, that the geomorphic signature of paleofloods is best preserved.
In arid regions, large floods can dominate fluvial landform evolution. Field sites in central Australia are examined to determine the flood signatures and chronology in canyons and unconfined reaches of rivers. This work aims to characterize the channel systems of large drainage basins in semi-arid environments and uses radiocarbon and OSL techniques to determine the timing of fluvial activity in the region. The data indicate a complex landform evolution with a hierarchy of fluvial forms embedded in the desert landscape (see Bourke and Pickup, 1999). The research has determined that there is a higher likelihood of preservation of fluvial deposits in the lower reaches of these systems (Bourke, 1999). It also assessed the long-term landscape response to flood events such as subsequent drainage development and aeolian reworking (Bourke, 1997; Bourke, 1998). Models developed for unconfined channel reaches in Australia have been applied to channels on Mars (see e.g., Bourke and Zimbelman, 2000; Bourke and Zimbelman, 2001)
Figure 2: An enhanced false colour MSS satellite image of the Hale River in central Australia.
The river flows from the McDonnell Ranges (shaded dark gray) at the top of the image into the longitudinal sand dunes of the northern Simpson Desert (shaded blue).
The area where the flood has deposited sediment into the dune field is well defined (shades of yellow to orange to brown to purple) by different vegetation types.
Satellite images allow unprecedented access to locations on the Earth’s surface that have traditionally been poorly studied because of their remote location. Satellite data can be processed to identify different landforms (e.g., mountains, sand dunes, rivers) and can identify the deposits and channels of past floods. The enhanced Multi spectral scanner (MSS) satellite image shown in Figure 2 has been adjusted to highlight the different landforms of the lower Hale River in Central Australia. This remote sensing data was used in field expeditions to identify sites where past floods had deposited sediments. Once located, their position was recorded using a Global Positioning System. The channels were surveyed (Figure 3) to enable the estimation of the size of the flood and flood sediments were excavated and sampled for analysis in the laboratory (Figure 4 and 5). By concentrating our sampling design in the downstream, unconfined area of catchments we hope to sample flood deposits that are older than those found in the upstream gorges.
Figure 3 Surveying across the wide flood channel using a laser system.
Brian Garvey holds the reflective target during the survey.
In this figure the flood sediments are covered by a thin veneer of aeolian sands.
Figure 4 Flood deposits are excavated using pit exposures and sand augers
Figure 5 Typical flood channel sediment textures (gravel supported in a medium sand matrix).
River gorges (see Figure 6) do preserve spectacular evidence of large floods. For example, the presence of large boulder bars (Figure 7) and slack water deposits (Figure 8) testify to the potential preservation of evidence of floods in gorges but these records tend not to be very long. This is because in steep and confined reaches, the high energy of floods remove the fine grained deposits emplaced by previous floods. Downstream, in the unconfined locations the flood water moves laterally and flow energy decreases. This leads to extensive deposition of flood sediments over a wide area (see Figure 2). The preservation of the flood deposits is enhanced by the tendency of the channel to shift its location after a small number of floods and therefore abandon its former path.
Figure 6 The absence of large boulders on the channel floor that periodically fall from the gorge walls,
indicate the high energy flood flows through this reach of the Hale River gorge.
Figure 7 These large boulders have been transported by floods to a location downstream from the narrow,
high-energy reach of the Hale Gorge seen in Figure 6. View looking downstream, Dr. Blau as scale.
A sediment dating technique known as Optically Stimulated Luminescence estimates the time that has elapsed since quartz sand grains were exposed to the sun (i.e. the time since they were deposited and buried by the flood). This analysis has recently determined that two catastrophic floods occurred within the last 1,000 years in the Hale River. The young age of these large flood events is an important finding. It indicates that events of this magnitude have occurred in this region of Australia during the present climatic regime. Events of this magnitude are not in the instrumental record. This illustrates the importance of undertaking geomorphological studies to reconstruct flood magnitude and frequency in river catchments. These preliminary results agree with those of the adjacent Todd River where geologic data indicates that this region has been regularly subjected to catastrophic floods since about 27,000 years ago (Bourke, 1999).
Figure 8 Floods also carry fine sediment high in the water column and deposit it in caves, on ledges and in the mouths of tributaries.
These slack water deposits can be used to indicate the height of floods. This slack water deposit lies ~10 m above the channel bed and
extends ~75 m up the tributary. The date on Fox’s grave indicates that this site has not been inundated by a similarly large flood in ~160 years.
Figure 9 Dr. Soren Blau (Australian National University) and Brian Garvey (Sheffield University).
Acknowledgements: Dr. Soren Blau (Australian National University), Brian Garvey (Sheffield University), Ken Meegan, Lisa Sedger and Kathleen Bourke are thanked for their assistance in the field.
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Bourke, M.C., (1998) Channel Adjustment to Extreme Events in Central Australia. In: E. Bliss (Editor), Proceedings of the Institute of Australian Geographers and New Zealand Geographical Society Second Joint Conference, Hobart, Australia, 1997. New Zealand Geographical Society Conference Series. New Zealand Geographical Society, Palmerston North, New Zealand, pp. 488-492.
Bourke, M.C. and Pickup, G., (1999) Fluvial form variability in arid Australia. In: A. Miller and A. Gupta (Editors), Varieties of Fluvial Form. Wiley, pp. 249-271.
Pickup, G., Marks, A., and Bourke, M. (2002). Paleoflood Reconstruction on Floodplains Using Geophysical Survey Data And Hydraulic Modeling. In Ancient Floods Modern Hazards: Principles and applications of paleoflood hydrology. (P. K. House, R. H. Webb, V. R. Baker, and D. R. Levish, Eds.), pp. 47-60. Water Science and Application. American Geophysical Union, Washington DC.
Bourke, M.C., (2004) Scabland, channeled scabland. In: A. Goudie (Editor), Encyclopedia of Geomorphology. Routledge, London, pp. 912-914.
Bourke, M.C., 1999. Fluvial geomorphology and paleofloods in arid central Australia. Ph.D. Thesis, Australian National University, Canberra, 208 pp.
Bourke, M. C. (1994). Geomorphic effects of Holocene paleofloods in central Australia: catastrophic floodplain processes. In “International Association of Hydrological Sciences.” Canberra.
Bourke, M. C. (1995). Complex landform assemblages of the Todd River in semi-arid central Australia. Proceedings of the International Association of Geomorphologists South East Asia Conference, Singapore.
Bourke, M.C. (1995) Geomorphic Effects of Holocene Super Floods on the Todd River, Semi-Arid Central Australia, in Proceedings of the XIV INQUA Congress, Berlin.
Bourke, M. C. (1997). The geomorphic effects and chronology of extreme floods in central Australia. Australia-New Zealand Geography Conference, Tasmania.
Bourke, M. C. (1997). Quaternary floods in Central Australia. CLIMANZ IV, Canberra, Australia
Bourke, M. C., Spooner, N. A., and Chappell, J. (1999). Late Pleistocene flood records from central Australia. International Quaternary Congress, Durban, South Africa.
Bourke, M. C., Spooner, N. A., and Chappell, J. (1999). Paleoflood record beyond the gorge in central Australia. The second international paleoflood conference. Prescott, Arizona.
Bourke, M. C. (2000). Paleoflood records in unconfined channels in Central Australia. South African Association of Geomorphologists .Johannesburg, South Africa.
Bourke, M.C. and Zimbelman, J.R., (2000) Australian paleoflood systems: An analogue for Martian channel systems, XXXI Lunar and Planetary Science Conference
Bourke, M.C. and Zimbelman, J.R., (2001) The Australian paleoflood model for unconfined fluvial deposition on Mars, XXXII Lunar and Planetary Science Conference.