A waituna is a freshwater coastal

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...

on a mixed sand and gravel (MSG) beach, formed where a

braided river A braided river (also called braided channel or braided stream) consists of a network of river channel (geography), channels separated by small, often temporary, islands called ''braid bars'' or, in British English usage, ''aits'' or ''eyots''. ...

meets a coastline affected by

longshore drift Longshore drift from longshore current is a geological process that consists of the transportation of sediments (clay, silt, pebbles, sand, shingle, shells) along a coast parallel to the shoreline, which is dependent on the angle of incoming w ...

. This type of waterbody is neither a true lake, lagoon nor estuary. This classification differentiates it from hapua, a type of river mouth lagoon. Both waituna and hapua are rare globally but common in

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

, where they are considered ecologically significant as sites of traditional

Māori Māori or Maori can refer to: Relating to the Māori people * Māori people of New Zealand, or members of that group * Māori language, the language of the Māori people of New Zealand * Māori culture * Cook Islanders, the Māori people of the Co ...

food-gathering as well as for their diversity of fish and bird species. Waituna form an interlinked chain of habitats which run the length of the east coast of the

South Island The South Island ( , 'the waters of Pounamu, Greenstone') is the largest of the three major islands of New Zealand by surface area, the others being the smaller but more populous North Island and Stewart Island. It is bordered to the north by ...

: from Wairua Lagoon and Lake Grassmere / Kapara Te Hau in

Marlborough Marlborough or the Marlborough may refer to: Places Australia * Marlborough, Queensland * Principality of Marlborough, a short-lived micronation in 1993 * Marlborough Highway, Tasmania; Malborough was an historic name for the place at the sou ...

, through

Lake Ellesmere / Te Waihora Lake Ellesmere / Te Waihora is a shallow coastal lake or waituna, in the Canterbury, New Zealand, Canterbury region of the South Island of New Zealand. It is directly to the west of Banks Peninsula, separated from the Pacific Ocean by the long, ...

and Coopers Lagoon / Muriwai in central

Canterbury Canterbury (, ) is a City status in the United Kingdom, city and UNESCO World Heritage Site, in the county of Kent, England; it was a county borough until 1974. It lies on the River Stour, Kent, River Stour. The city has a mild oceanic climat ...

, and

Washdyke Lagoon Washdyke Lagoon (known in Māori as Waitarakao) is a brackish shallow coastal lagoon approximately north of Timaru, South Canterbury, New Zealand. The lagoon has drastically reduced in size since 1881 when it was approximately , now it is less th ...

and

Wainono Lagoon Wainono Lagoon is a shallow lagoon in the southern Canterbury region of New Zealand's South Island. A number of rivers flow into the lagoon, including the Hook River from the north and the Waihao River from the south. Nearby settlements include ...

in South Canterbury, to

Waituna Lagoon __NOTOC__ The Waituna Lagoon is on the southern coastline of the South Island of New Zealand. It forms part of the Awarua Wetland, a Ramsar site that was established in 1976. It gives its name to waituna, a type of ephemeral coastal lake. The ...

in Southland.

is one of the finest examples. The name translates as “water of eels”.

Difference between a hapua and a waituna

Both waituna and hapua are fed by small rivers flowing out to an MSG beach, on an eroding wave-dominated coast, with strong longshore drift. Hapua are long-lived river mouth lagoons, whereas waituna are

ephemeral Ephemerality (from the Greek word , meaning 'lasting only one day') is the concept of things being transitory, existing only briefly. Academically, the term ephemeral constitutionally describes a diverse assortment of things and experiences, fr ...

coastal "lakes" that do not persist over geological time. Hapua persist because the river mouth turns sideways and carves out cliffs on a daily basis, whereas waituna form when gravel creates a barrier that impedes the drainage of freshwater. Natural openings through the barrier are rare, although artificial openings are common. Over geological time, provided natural coastal processes are able to operate unimpeded, the "lake" floods and infills, or the barrier disappears with coastal erosion.

Waituna characteristics

Waituna are low in salt, high in water that lies around for a long time, and shallow – typically less than deep. They are tideless and the water is not well mixed by wind. The barrier prevents flushing by rainfall events. All this is evident in the green colour of water in above photo.

Waituna case study

Te Waihora / Lake Ellesmere is an outstanding natural feature. It is a wetland of international importance and an area of immense cultural significance to

Ngāi Tahu Ngāi Tahu, or Kāi Tahu, is the principal Māori people, Māori (tribe) of the South Island. Its (tribal area) is the largest in New Zealand, and extends from the White Bluffs / Te Parinui o Whiti (southeast of Blenheim, New Zealand, Blenhe ...

. The waituna is fed by two small rivers,

Selwyn River / Waikirikiri The Selwyn River (, officially gazetted as Selwyn River / Waikirikiri) flows through the Selwyn District of Canterbury in the South Island of New Zealand. Description The river has its source in the Southern Alps and flows east for before e ...

and Irwell River. These bring it a, comparatively, low volume of water, along with loads of silt and sand. At the coast, in an area known as

Canterbury Bight The Canterbury Bight is a large bight on the eastern side of New Zealand's South Island. The bight runs for approximately from the southern end of Banks Peninsula to the settlement of Timaru and faces southeast, exposing it to high-energy storm ...

, a current runs parallel to the shore from south to north. This strong current brings with it sand and gravel sourced from the

Rakaia River The Rakaia River is in the Canterbury Plains in New Zealand's South Island. The Rakaia River is one of the largest braided rivers in New Zealand. The Rakaia River has a mean flow of and a mean annual seven-day low flow of . In the 1850s, Europ ...

. The bight is chronically eroding because its beaches are starved of sediment. The Rakaia River, incidentally, has a hapau at its mouth. Over geological time, a spit grew, drift-aligned to the current. It was, initially, as to be expected, wide the southern end where it receives the most sand and gravel, and narrow at the northern end. However, as it grew, it bumped into

Banks Peninsula Banks Peninsula () is a rocky peninsula on the east coast of the South Island of New Zealand that was formed by two now-extinct volcanoes. It has an area of approximately . It includes two large deep-water harbours — Lyttelton Harbour a ...

. This stopped it growing, but the sediment kept coming, so it no longer fits the rules of a spit. It's now wider at the

proximal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

end and narrower at the

distal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

end. It's become a barrier. Approximately 5000 years ago, it swung around, hinged at the point near Wairewa

marae A ' (in Māori language, New Zealand Māori, Cook Islands Māori, Tahitian language, Tahitian), ' (in Tongan language, Tongan), ' (in Marquesan language, Marquesan) or ' (in Samoan language, Samoan) is a communal or sacred place that serves reli ...

on the shores of Lake Forsyth / Waiwera. The barrier, named

Kaitorete Spit Kaitorete Spit is a long finger of land which extends along the coast of Canterbury, New Zealand, Canterbury in the South Island of New Zealand. It runs west from Banks Peninsula for , and separates the shallow Lake Ellesmere / Te Waihora from th ...

, even though it's a barrier not a spit, prevents the Selwyn / Waikirikiri and Irwell rivers from draining into the sea, and hence, a waituna sits behind the barrier. Its natural lake level is above

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

Photographs

File:Cape Campbell sunrise.jpg, alt=, Lake Grassmere / Kapara Te Hau File:Washdyke Lagoon.JPG, Washdyke Lagoon File:Lake Ellesmere sunset.jpg, alt=, Te Waihora File:ISS023-E-28547 - View of the South Island of New Zealand.jpg, alt=, Wairau Lagoon

References

{{coastal geography * Fluvial landforms