Catchment Description by lindayy

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									2.0 CATCHMENT DESCRIPTION

Introduction

The Darwin Harbour region is bounded by Charles Point in the west, Gunn Point at
the eastern end of Shoal Bay and the catchments of creeks and rivers flowing into the
harbour (Map 1.1, p.2). The Darwin Harbour region covers an area of 3230 km2 with
a land area or catchment of 2010 km2 and estuary area of 1220 km2 to the high water
mark.

The catchment to estuarine ratio for Darwin Harbour is approximately 3:1, which is
relatively small. This means that the area that can generate potentially polluted run-off
that flows into the harbour is less than for other proportionally larger catchments, such
as Brisbane’s Moreton Bay or Sydney’s Port Jackson. Darwin Harbour has naturally
deep channels that are 10 to 12 m deep and extends into the three major arms: East
Arm, Middle Arm and West Arm.

The tidal range in the harbour is up to 8 m, and referred to as macro-tidal. There are
two tides every day, with mean spring tides of typically 6 m and neap tides about 3 m.
The large tides produce strong currents that peak at 2.5 metres/second, and transport
large volumes of water. Over a spring tide, as much as one billion m3 (or tonnes) of
water can pass though a line between East Point and Mandorah. Unlike Port Darwin,
Shoal Bay is a shallow embayment. The harbour is fringed by mangroves, mud flats,
rocky foreshores, low cliffs and sandy beaches.

The catchment’s geology is ancient and highly weathered, consequently most soils in
the region have poor fertility. The topography of the catchment is relatively low-lying
with most land being less than 30 metres above sea level. Inland plains are flooded
with fresh water to a depth of up to two metres each wet season.

For a discussion of the aquatic flora and fauna of the Darwin Harbour region see
sections in this report on the Freshwater Aquatic Environment, macroalgae , macro-
invertebrates, fish, diatoms, mangroves, and stream riparian vegetation.


Human Impact in the Catchment

Population pressure and development have
shaped land-use in the Darwin Harbour region.
Approximately 110 000 people, including 1 600
indigenous Larrakia people live in the Darwin
Harbour region. The population is primarily
located in the cities of Darwin and Palmerston,
with smaller rural townships in the Litchfield
and Coomalie Shires to the south and Cox
Peninsula Shire in the west. Darwin’s central
business district is located on the edge of the
                                                           Figure 2.1 View of Darwin City
harbour with residential development extending             from Charles Darwin National Park
towards the north and east (Figure 2.1).


The Health of the Aquatic Environment in the Darwin Harbour Region                        3
Industry is located primarily in the suburb of Winnellie fringing the eastern margin of
Port Darwin. However, a number of satellite industrial estates service the East Arm
Port region at Hudson Creek, whilst others are located in the Berrimah and Pinelands
areas. More recently, approximately 88 hectares of Wickham Point has been cleared
to establish an LNG plant and its associated infrastructure. Industrial and
manufacturing land-use, including the port, constitutes 0.15% of land-use in the
catchment or approximately 990 ha.

Approximately two percent of land-use in the catchment has been developed for
horticulture. These areas are located primarily in the Litchfield and Coomalie Shires.
Intensive horticulture only occupies some 93 ha of the catchment or 0.01%.

The impact of urban land-use on the water quality entering the harbour is described in
Sections 6 and 8.


Clearing in the Catchment

Approximately 19% or 46,000 ha of native vegetation has been cleared in the Darwin
region. Clearing in the catchment has provided living space, roads, homes and other
services for the region’s population. It has been estimated that the Darwin and
Palmerston urban centres account for 10,000 ha of total cleared land in the catchment,
and that in the last 25 years just under 19,000 ha has been cleared. Outside of the
major urban areas, the tenure most affected by clearing is freehold land where rural
residential, agricultural and horticultural land-uses predominate (Map 1.1, p.2).

Based on Department of Natural Resources, Environment and the Arts estimates from
satellite data, clearing rates were steady between 1977-1990 with an average rate of
460 ha/yr, decreasing over 1990-95 to 210 ha/yr, and increasing to 1145 ha/yr from
1995-2000. A recent investigation suggests that figures have returned to pre-1990
levels.


Land Tenure and Land-use Characteristics

The most common tenure type in the Darwin Harbour catchment is freehold and
vacant Crown land. It is freehold land, however, that has been the focus of clearing
with two-thirds or about 13% of the catchment area having been modified.

Freehold land has been cleared for urban living, rural residential living, horticulture,
agriculture, infrastructure, defence facilities and manufacturing. Clearing of land
under pastoral tenure is minor. Clearing for extractive mining for sand, gravel soil and
rock is very small, and occurs mostly outside the catchment area.




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Riparian Environments

Riparian land is the land that borders rivers and streams and is usually distinguished
by its own characteristic vegetation. This land is especially important to maintaining
the health of rivers, as described in more detail in Section 4. To evaluate the extent of
riparian clearing in the Darwin region, information about waterlogged soils has been
used as a surrogate for riparian land. Approximately 6846 ha or 10.7% of severely
waterlogged areas have been cleared of native vegetation. This includes some minor
riparian tracts and other seasonally inundated areas that have been cleared to
accommodate urban and rural residential development, industry and infrastructure.


Conclusion

Human impacts and land-use in the catchment can have significant bearing on the
health of the aquatic environment. The Darwin Harbour region is not intensely
developed, with a large portion of the region still not cleared. Moreover, the region’s
catchment is small compared to the region’s estuary. Nevertheless, with increasing
population and development there will be greater pressures on the region’s
environment. Monitoring land-uses and human activities in the region provides
important information to help manage and maintain the health of the aquatic
environment.


Further Reading

Darwin Harbour Advisory Committee (2003). Darwin Harbour Regional Plan of
Management. Department of Infrastructure, Planning and Environment, Darwin.

Department of Infrastructure, Planning and Environment (2001). Land-use mapping
of the Northern Territory. Department of Infrastructure, Planning and Environment,
Darwin.

Working Group to the Darwin Harbour Advisory Committee (2003). Management
Issues for the Darwin Harbour Region. Natural Resource Management Division,
Department of Infrastructure, Planning and Environment, Darwin.




The Health of the Aquatic Environment in the Darwin Harbour Region                     5
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