MUDDY FEET - Auckland Regional Council

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The Firth’s geographical position makes it vulnerable to changes induced by natural events
and human developments. The extent of impact arising from such events has, over the years,
attracted considerable attention from numerous scientists and amateur naturalists. Land use in
the catchment is predominantly agricultural, and recent years have seen intensified agriculture
on the Hauraki Plains, accelerated growth of the Auckland and Waikato Regions, and the
establishment of the Pacific Coast Highway around the perimeter of the Firth of Thames. This
has resulted in ever increasing pressure on the environmental quality of this ecosystem. The
Firth’s role as the receiving environment for the run-off from most of these areas also makes
it a potentially very important site for monitoring of critical environmental indicators
(Ministry for the Environment, 1997).
1.4 Catchment
The Firth receives freshwater influence directly from two main rivers: the Waihou and the
Piako (Figures 1-2 and 1-3), the smaller Waitakaruru River, the Miranda Stream, Karito
Canal, and a few lesser streams and drains in the west. The Waihou catchment extends back
as far as Putaruru and together with the Piako drains nearly all of the Hauraki Plains.
Historically this was covered with a vast kahikatea forest but is now developed for farming
and a small amount of forestry. Within Hauraki District 67% of land is in primary
production. The Waihou and Piako together contribute most of the sediments and nutrients
that run off the land into the Firth of Thames. Much of the remainder of the catchment in
Hauraki District lies within the Kopuatai peat dome - itself a Ramsar site and a major
component of the Hauraki Plains Flood Control Scheme. There are also other small areas of
peat bog and kahikatea forest under conservation management in the Waihou and Piako
catchments.
The wider Firth of Thames receives freshwater runoff from the Hunua and Coromandel
Ranges - both largely vegetated in indigenous forest. Much of the Hunua water is diverted
into large water supply dams serving Auckland, but flood flows from Coromandel rivers (in
particular from the Kauaeranga, Te Puru, Waiomu, Tapu and Te Mata Rivers) can have a
strong (if temporary) influence contributing to flooding of coastal settlements and sediment
loading in the Firth.
1.5 The Firth of Thames Ramsar Site
The Firth of Thames Ramsar Site is comprised of intertidal area of the southern and western
shores of the Firth of Thames between the west bank of the Waihou River and Kaiaua (Figure
1-1and Figure 1-3). The intertidal area is defined by the extremes of mean low water spring
tides (MLWS), and mean high water springs (MHWS), and covers about 8,500 ha (Veitch &
Habraken, 1999). The vast, open inter-tidal mudflats, the mangrove communities that now
cover much of the upper reaches of the intertidal zone, and the terrestrial areas adjacent to the
mudflats are built on varying proportions of marine and land-derived sediments, old shell
banks and vegetative debris.
The graded shell banks of the coast between Miranda and Whakatiwai (Figure 1-1, Figure 6-3
and Figure 9-2) are mainly derived from an active Chenier Plain, gradually pushed up on the
shores by northeasterly storms and high tides. The Miranda Chenier Plain (course sediment
and/or shell banks over-laying finer sediments) is unique in New Zealand, and one of the
world’s finest examples of this rare coastal landform (Woodroffe, et al, 1983). A series of
true Chenier ridges exist on the stretch of foreshore between Miranda and Kaiaua on the west
coast of the Firth. This site has been the focus for a number of national and international
EcoQuest Education Foundation EQRS/1 12/04