Floodplains as dynamic mosaics : sediment and nutrient patches in a large lowland riverine landscape

Abstract

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 (

Date of Award	2008
Original language	English
Supervisor	Martin THOMS (Supervisor), Jon M. Olley (Supervisor) & Michael A. Reid (Supervisor)