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Introduction
As part of the water cycle, rivers drain freshwater runoff from rainfall events over
land. The runoff collects a variety of substances as it moves through the river’s
catchment-lands and waterways including nutrients, sediments and contaminants
depending on the catchment characteristics and land-use practices. Upon reaching
the sea at the river’s mouth, the runoff drives a buoyant plume into coastal and
shelf waters. The plume eventually spreads and mixes and moves around with the
winds and currents. This mixing with ambient coastal waters will ultimately dilute
the runoff plume as well as any concentrations of sediments, nutrients and
contaminants carried within the plume.
In the wet and dry tropical catchments adjoining the Great Barrier Reef (GBR),
river discharges are highly seasonal and usually event-driven in nature and result
from rainfall events associated with evolving monsoon troughs or passing tropical
cyclones (Furnas and Mitchell 2000; Wolanski, 1994). Also, the unpredictable
nature of rainfall and runoff events, and the unsteadiness and patchiness of the
resulting plume intrusions in a complex region such as the GBR, has traditionally
made data logistically difficult to collect. Further, direct rainfall inputs onto the
shelf-waters can also lower surface salinity significantly (Wolanski, 1994) and at a
time when river discharges are also significant. Thus large spatial mapping of
salinities was needed to determine the origins of lower salinity events (< 34 ppt)
within the reef matrix of the GBR.
Historically, significant plume intrusions into the GBR have been observed. Data
presented in the literature (Wolanski et al., 1997; Ayukai et al., 1997; Wolanski,
1994; O’Neill et al., 1992; Wolanski and Ruddick, 1981) report measurements of
reduced salinities associated with particular events. Wolanski and Van Senden
(1983) reported the most detailed survey todate, which covered the 1981 flood
events from the Burdekin, Herbert, Tully, Johnstone and Barron Rivers. These
studies have also provided some insight into the dynamics influencing the fate of
river plumes in the GBR. However, understanding all the possible fates of river
plumes under a variety of climatic conditions is needed for the management of the
GBR because of their ability to transport pollutants from human activities on land
into the GBR Marine Park. Hence, information on the fate of plumes over daily to
decadal time scales is ultimately required to comprehensively determine the full
range of impacts from river plumes in the GBR.
The aim of this current project was to utilise these historical observations to
calibrate a 3-dimensional hydrodynamic plume model of the Burdekin, Herbert,
Tully, and Johnstone Rivers in flood (see location map in figure 1). The model’s
predictions have been verified against field observation where possible (See also
King et al., 1998). The model was used to produce a comprehensive long-term time
varying and 3-dimensional spatially varying database of the fate and mixing of
plume waters from the Burdekin, Herbert, Tully and South Johnstone Rivers from
1973-1998 (McAllister et al., 2000).
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