Identifying relationships between flood history, flood frequency and the provenance of surface sediments in a semi-arid terminal wetland

Abstract

In semi-arid environments, dryland wetlands serve as key loci for biological diversity and productivity. This stems from their relative abundance of water and the comparative richness of their soils which are reinvigorated by the delivery of sediment and nutrients during relatively infrequent flood events. Therefore, to fully understand the nature of these environments, it is important to understand the links between the delivery of water and sediment (particularly with respect to varying sediment sources) to semi-arid wetlands and the physical and chemical properties of the surface sediments deposited within them. The purpose of this study was to: (1) determine the provenance (e.g. locally derived or fluvial sources) of surface sediments within a semiarid wetland, the Narran Lakes Ecosystem in central eastern Australia; (2) determine how sediment provenance relates to flood frequency and flood history; and (3) identify variations in the physical and chemical properties of sediments with different sources. The study employs a set of 163 samples, collected along an irregular grid spaced at ~1.8 km, which were analysed to determine the physical and chemical properties of the surface sediments. The ratio of titanium to aluminium (Ti/Al) was used to differentiate between fluvial sub-catchment and locally derived sediment sources. The sourced sediments were then compared to flood frequency maps in the wetland and related to the flood history of the two principal source sub-catchments to see if sediment sourcing could be reliably linked with long term flood inundation patterns. The results of this study indicate that there are distinct and strong associations between the source of the sediments in a particular location of the wetland and the frequency of flooding that occurs there. These associations can be more completely understood by examining the flood history of the source sub-catchments. In addition, the sediments derived from each fluvial source and from locally derived hillslopes have distinct differences in their physical and chemical properties. Thus, the nature of the sediments and the resultant ecology of the Narran Lakes Ecosystem may be influenced by differential sediment sources.