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Overview
Tasmania is the most coastal state in Australia, with the longest coastline
length relative to its land area (Zann 1995). The coastal, estuarine and
marine environs covered by the Region extend from Badger Head in the west
to West Sandy Point in the east, along the north coast of Tasmania and
include the Tamar Estuary, North Esk River to Hoblers Bridge and South
Esk River to the Basin. These areas encompass rocky reefs, sandy beaches,
headlands, the Tamar River Estuary, harbours and open coast.
The head of the Tamar River Estuary is at Launceston, formed by the confluence
of the South Esk and North Esk Rivers. Covering near 100 square kilometres,
it is one of the State's larger estuaries, extending approximately 70
km from the City of Launceston to Bass Strait. The Tamar is a narrow estuary
with a deep, well-defined channel, bordered by shallow tidal mud flats
and wetlands. The Estuary has a three metre tidal range with fresh water
inputs from the North and South Esk Rivers and numerous rivulets (See
Plate 7). Combined with the geomorphology of the Estuary, large sediment
loads and strong tidal currents have resulted in the sedimentation of
the upper reaches of the Estuary.
However, siltation in the Tamar Estuary is a highly complex issue. Many
hypotheses have been put forward as to where the particles come from and
how they move within the system.
These include:
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Sedimentation of the estuary is a natural process which has been
exacerbated through urban development, reclamation and land use within
the catchment.
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Sedimentation of the estuary has resulted from poor land management
practices, both agricultural and forestry, which have contributed
to the gross sediment load within the system. Clearance of native
vegetation along the banks of rivers and adjacent areas within the
Tamar's catchment, particularly the South Esk and Meander River basins,
has been a major contributor to sediment problems.
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Sedimentation of the Estuary is resultant from irrigation, dams and
the diversion of water altering natural river flows and reducing flushing
of the system, thereby allowing for the build up of sediment within
the estuary.
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Sedimentation is a natural process and the estuary is returning to
its pre-navigation and dredging condition. Records show that the original
regime for the estuary was a narrow channel between mud banks.
The major sediment deposits within the Estuary are at Stephenson's bend,
upstream of Tamar Island (opposite North Riverside), at Home Point and
on the inside bends of the Estuary at Tail Race.
Plate 7: Catchments of the Tamar Estuary Reproduced
Courtesy of DPIWE
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The Estuary itself has been identified as possessing an exceptionally
high diversity for both fishes and invertebrates. The Tamar has been assigned
the highest conservation value ranking - Class A - by Edgar et al 1998.
The Estuary and its catchment area (sic) show minimal effects of human
activity and are identified as key components within an integrated system
of representative reserves around Tasmania. Class A also includes sites
with exceptional fish and invertebrate biodiversity. Plants, animals and
habitats within Class A estuaries and associated catchments should be
fully protected as a matter of urgency (Edgar et al. 1998).
The classification of the Tamar as an 'A Class' Estuary was considered
problematic. The Estuary was recognised as having the highest conservation
significance because it is the only mesotidal drowned river valley in
Tasmania and it possesses a large component of species not recorded elsewhere
in the State. However, the Estuary is also badly degraded as a result
of human activities, particularly the effects of the large urban population
in the lower catchment and land clearance in the upper catchment. Extremely
high loads of silt continue to be deposited in the Estuary, leading to
active dredging to maintain shipping channels and rising mudflats along
the shoreline. The Estuary is also badly affected by introduced species,
including rice grass, Spartina anglica, the east Asian bag mussel, Musculista
senhousia, and the pacific oyster, Crassostrea gigas.
Sedimentation of coastal, estuarine and marine systems is most notable
on the Tamar Estuary and North Esk River to Hoblers Bridge. The Tamar
Estuary receives inputs of sediments from the catchments of the South
and North Esk Rivers, the Macquarie River and the Meander River (See Plate
8: North Esk River at Blessington showing sediment build up). Due to estuarine
hydrodynamics, fine grained silts accumulate and are deposited in the
upper reaches of the Tamar Estuary. Rapid siltation in the Home Reach
section of the Tamar causes difficulties with navigation and restricts
recreational and tourism pursuits (Pers Comm J. O'May & R. Oliver, 1998).
Causes of this sedimentation have been linked to the clearance of land
for agriculture and forestry operations in the upper catchment. These
practices can disturb soils, resulting in their washing into water courses
that are part of the Tamar's catchment.
Plate 8: North Esk River at Blessington
showing sediment build up Reproduced courtesy of DPIWE
Sediment is released into water systems by stock accessing streams and
rivers, predisposing banks to erosion. The animals can disrupt the banks
and beds, releasing particles, including soils and nutrients. The expansion
and development of urban areas and associated construction sites add to
the net sediment loads entering the Tamar Estuary.
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