Sediment source changes over the last 250 years in a dry-tropical catchment, central Queensland, Australia

Abstract

Rivers draining to the Great Barrier Reef are receiving increased attention with the realisation that European land use changes over the last ∼ 150 years may have increased river sediment yields, and that these may have adversely affected the reef environment. Mitigation of the effects associated with such changes is only possible if information on the spatial provenance and dominant types of erosion is known. To date, very few field-based studies have attempted to provide this information. This study uses fallout radionuclide ( 137Cs and 210Pb ex) and geochemical tracing of river bed and floodplain sediments to examine sources over the last ∼ 250 years for Theresa Creek, a subcatchment of the Fitzroy River basin, central Queensland, Australia. A Monte Carlo style mixing model is used to predict the relative contribution of both the spatial (geological) sources and erosion types. The results indicate that sheetwash and rill erosion from cultivated basaltic land and channel erosion from non-basaltic parts of the catchment are currently contributing most sediment to the river system. Evidence indicates that the dominant form of channel erosion is gully headcut and sidewall erosion. Sheetwash and rill erosion from uncultivated land (i.e., grazed pasture/woodland) is a comparatively minor contributor of sediment to the river network. Analysis of the spatial provenance of floodplain core sediments, in conjunction with optical dating and 137Cs depth profile data, suggests that a phase of channel erosion was initiated in the late nineteenth century. With the development of land underlain by basalt in the mid-twentieth century the dominant source of erosion shifted to cultivated land, although improvements in land management practices have probably resulted in a decrease in sediment yield from cultivated areas in the later half of the twentieth century. On a basin-wide scale, because of the limited spatial extent of cultivation, channel sources are likely to be the largest contributor of sediment to the Fitzroy River. Accordingly, catchment management measures focused on reducing sediment delivery to the Great Barrier Reef should focus primarily on decreasing erosion from channel sources.