Late Quaternary stream channel adjustment in hydrologically variable catchments, subtropical Australia

Abstract

In regions of high climate and hydrological variability alluvial rivers exhibit complex arrangements between channel characteristics and flow regimes. Along the eastern margin of Australia, hydrologic variability reaches a peak in the subtropics of Southeast Queensland (SEQ), and many alluvial rivers have entrenched channels. These entrenched channels are termed ‘macrochannels’ which accommodate large floods and feature a suite of inset alluvial units. However, the formation of the macrochannels and their particular relation to hydrological regimes remains largely unresolved. This study investigates whether the macrochannels of SEQ are related to the contemporary and variable flow regimes or are inherited late Quaternary landforms. The aim of this thesis is to address the nature of channel entrenchment and relationships with adjacent alluvium and flood hydrology. Geomorphological, hydraulic and chronostratigraphic data is evaluated from alluvial settings within SEQ, with a detailed site investigation presented in the mid-reaches of Lockyer Creek.

This research examines the hydrology and geometry of 65 gauging stations in alluvial reaches within SEQ. No common bankfull inundation frequency was found and annual exceedance probabilities (AEPs) range from approximately 1.5 - >100 years, with a median of 9.75 years. Macrochannels appear ubiquitous throughout the region, with >50% of alluvial rivers analysed exhibiting a bankfull capacity that exceeds the 10-year AEP. Macrochannel bankfull frequencies are broadly distributed, with a peak in channel capacity at the 20-year recurrence interval but an overall median of 45 years and some adjacent surfaces have never been inundated. At a regional level, macrochannel dimensions scale with catchment area but highly variable geometries bear little relationship to the modern flow regime or slope. Entrenchment has mostly occurred in dryer, western catchments with a higher degree of hydrological variability, despite no statistically significant correlation between flow variability and the degree of entrenchment. Nonetheless, their presence serves to constrain most contemporary river flows.  

Despite high hydrological variability throughout the region and considerable variation in macrochannel capacities, 95% of streams feature active ‘inner’ river channels constructing and inundating adjacent alluvial units at the mean annual flood (MAF). However, only 25% of streams analysed engage the main valley floor surface as their primary floodplain. Holocene alluvium is mostly present as either a narrow, depositional wedge in the form of inset floodplains and benches or as a drape overtopping buried terraces.  Over 90% of entrenched streams have compound forms featuring inset alluvial units which range from several metres to 150 m wide within the macrochannel.

In the Lockyer Creek catchment, downstream macrochannel geometry is highly variable at the reach scale and in part has been influenced by antecedent structural/lithological controls and the presence of resistant valley fills. Abrupt morphological changes and variation in macrochannel geometry coincide with changes in sinuosity and confinement against the valley margin. However, an overall downstream decrease from mid-catchment peaks in macrochannel area are apparent in all catchments and correspond with increased preservation of extensive fill terraces that occupy 70-95% of mid-lower catchment valley floors.

Terrace sediments are spatially variable but consist of 5-12 m of highly weathered heavy clays to fine-sandy clay loams. Radiocarbon and single grain optically stimulated luminescence (OSL) dating indicate a large proportion of the valley fill reflects a significant phase of aggradation of fine-grained alluvium from ca. 35 ka throughout the Last Glacial Cold Period (LGCP) that overlies older Pleistocene basal sediments. Subsequent channel incision up to 10 m is well-constrained in the mid-reaches of the Lockyer by upper terrace ages of 9.25 ± 0.84 ka to 11.5 ± 1.29 ka at four sites and basal floodplain ages of 10.08 ± 1.02 – 7.97 ± 1.03 ka. Surficial ages of terraces adjacent to macrochannels in three other key catchments across SEQ indicate synchronous, regional incision of Pleistocene alluvial fills at 9.24 ± 0.93 ka, with all age ranges within 1σ errors. Correlation with other regional chronologies and climate proxy records suggest the current entrenched river systems within SEQ formed in response to changes in late Quaternary climate, related to an early Holocene pluvial episode identified elsewhere in eastern Australia.

Holocene floodplains set within the entrenched Pleistocene valley floor have basal ages that are ca. 7.5 ka but whose proximal margins are still actively accreting. This Holocene fill has primarily accreted over the mid- to late-Holocene but overlaps with the contemporary hydrological regime. The dominance of the macrochannel form appears to be related to persistent inherited controls in the form of bedrock and terrace constriction, coupled with low rates of sediment supply and ongoing high hydrological variability. Fine-grained, mechanically resistant Pleistocene alluvium has imposed substantial controls on the formation of the widely distributed macrochannels of SEQ.

Advisory Team

Principal Advisor - Professor Jamie Shulmeister

Associate Advisors - Associate Professor Jacqueline Croke; Dr Timothy Cohen

Venue

Room 388, Goddard Building (#08)