HR: 1330h
AN: H42C-1084    [PDF]
TI: Fluvial Responses to Holocene sea Level Variations Along the Macdonald River, New South Wales, Australia
AU: * Rustomji, P
EM: paul.rustomji@csiro.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: * Rustomji, P
EM: paul.rustomji@csiro.au
AF: CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601 Australia
AU: Chappell, J
EM: john.chappell@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Olley, J
EM: jon.olley@csiro.au
AF: CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601 Australia
AB: The Macdonald River drains the rugged eastern flanks of Australia's Great Dividing Range. It has a catchment area of 2000km$^{2}$, restricted alluvial lowlands confined by bedrock interfluves and flows into the Hawkesbury River, a larger estuarine valley. The Macdonald valley is presently tidal for 14km from the Hawkesbury. At about 8000 year before present (BP), rising sea level invaded the Macdonald Valley for at least 35km upstream of the Hawkesbury River. Rapid aggradation occurred between 8000 and 6000 years BP and a sand bed river was established in the Macdonald Valley, its mouth prograding rapidly towards the Hawkesbury. Little is known about the character of the sand bed river during the +2 meter sea level highstand occurring between 5000 and 4000 BP. However, from 3000 to 1500 BP when sea level was consistently at +1 to +1.5m, major floodplain and levee-like structures, now virtually inactive, were established. The bed is inferred to have been elevated above its present day level and consequently intersected mean sea level (MSL) downstream of its present location. This is consistent with reported sea levels at +1 to +2m above present levels for the New South Wales coast at this time. From 1500 years BP, local sea level fell rapidly to its present level. Aggradation of the levee crests ceased and sedimentation along the valley became restricted to aggradation of an inset floodplain, within the pre-1500 BP deposits. The channel contracted and the sandy river bed incised. An equivalent and synchronous change in sedimentation style is observed along the Tuross River 400km south of the Macdonald, lending support to sea level variations being the factor driving this change. By 1850 AD, the bed dipped below MSL about 10km upstream of its inferred position prior to 1500 years BP. A series of large floods between 1949 and 1955 eroded significant volumes of sandy sediment from the Holocene deposits. The channel bed widened from between 25 and 50m width to $\sim$100m along some 35km of river, mostly upstream of the tidal limit, and meanders were cut off. The bed aggraded by 3m and the point of intersection of the river bed with MSL shifted 7.6km down valley. The channel has contracted since these floods and is building a new floodplain below the post-1500 BP inset floodplain. The late Holocene fluvial history of the Macdonald River is one of rapid adjustment to falling base level. The formation of the major fluvial landforms of the valley coinciding with late Holocene sea level at +1 to +2m, followed by subsequent abandonment of this surface with associated bed incision and channel straightening, may be a common phenomenon along river systems of eastern Australia. If so it may explain the propensity for instability of rivers along the New South Wales coast.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 4235 Estuarine processes
DE: 4556 Sea level variations
SC: Hydrology [H]
MN: 2003 Fall Meeting