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larger valleys of Hoodoo and Sellars Creeks can become fully inundated to 1-2 km in width.
The higher plains, ridgelines and small oval wetlands (pans) on interfluves of the Balurga
land system might become saturated and filled from local rainfall, but they appear to be
rarely inundated by actual flood water from creeks. However, these remote sensing data are
also masked by forest cover > 30% in these areas (Figure 57).
The remotely-sensed distribution of floodwater was also compared to the remotely-sensed
distribution and dry-season visual clarity of both large “lacustrine” lagoons with permanent
water (billabong, oxbows, lakes) and smaller “palustrine” wetlands with intermittent to
permanent water (pans, depressions, swamps, bogs, marshes). The distribution (but not
clarity) of small palustrine wetlands > 0.2 ha was mapped by DERM (Queensland
Department of Natural Resources (QDERM) 2010). For more permanent wetlands,
(Lymburner and Burrows 2008) analysed Landsat TM satellite imagery between 1986 and
2005 across the Northern Gulf area. They mapped the distribution of both lacustrine lagoons
and larger palustrine wetlands (in-channel and off-channel) that contained open water and
typically do not dry out in the dry season. During each dry season year for the period, they
estimated the visual clarity (via colour) of the water body using the green-band digital number
from Landsat, correlated to some secchi depth measurements of light penetration and water
clarity. They classified clarity along a range between very clear and always turbid (Figure
57).
Comparing the distribution of flood waters to the distribution of lacustrine and palustrine
wetlands demonstrates that most of the wetlands located along major creek and river valleys
in Quaternary alluvium are regularly inundated (Figure 57). This is in contrast to the
numerous palustrine wetlands on ridgelines, interfluves, and higher plains of the area that
are not frequently flooded by creeks, but rather filled locally by rainwater and perhaps
groundwater.
Wet season water clarity, turbidity, or sediment concentrations measurements (or even
remotely sensed estimates) are not available for the area, and should be a priority in future
wet-season data collection. Comparisons of available dry-season clarity data (remotely
sensed) to wet-season flood distributions suggest that wetland waterbodies along major flood
paths in Quaternary alluvium are “usually turbid” during the dry season (Figure 57). This is
likely due to 1) the abundance of silts and clays transported in the wet season along these
paths that stay in suspension into the dry season (colloidal clays), 2) the re-suspension of
silts and clays in the dry season due to wind or local animal disturbance (cattle, pigs)
concentrated along the banks of water bodies, and/or 3) the greater availability of nutrients
for primary production (algae phytoplankton) along major watercourses, which all reduce
water clarity. In contrast, wetland water bodies that are adjacent to or outside of major flood
paths are typically “clear to turbid”, “usually clear”, or “always clear”, such as toward the edge
of Quaternary valleys or on higher elevation terraces or tertiary sediments. Sediment and
nutrient delivery to these wetlands may be lower or local hydrogeologic factors may influence
the delivery or filtration of sediment and nutrients. For example, sand bars lying along the
main Alice and Mitchell River channels filter out finer particulate matter during the dry
season base discharge, keeping in-channel pools clear in the dry season that become turbid
in the wet season (Figure 57).
On Oriners Station, locally important lagoons (Figure 4) such as Mosquito Lagoon along
Eight Mile Creek were mapped as “usually turbid”. In addition, Horseshoe Lagoon lying along
a tributary next to Crosbie Creek was classified as “usually clear”, despite this location being
inundated by floods 4 times out of 7 years. Oriners Lagoon unfortunately was not mapped or
classified, due to omissions or errors. These important errors and omissions in the remotely
sensed data indicate the need for ground validation and cross-referencing with data from
knowledgeable local people. Improvements in the understanding and management of these
important wetlands requires the combination of local knowledge and data with appropriate
field hydrology techniques, as well as the data provided by new scientific tools such as
remote sensing.
Working Knowledge at Oriners Station, Cape York
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