Lake Forsyth (known to Māori as Te Roto o Wairewa) is a lake on the south-western side of

Banks Peninsula Banks Peninsula is a peninsula of volcanic origin on the east coast of the South Island of New Zealand. It has an area of approximately and encompasses two large harbours and many smaller bays and coves. The South Island's largest cit ...

in the Canterbury region of New Zealand, near the eastern end of the much larger

Lake Ellesmere / Te Waihora Lake Ellesmere / Te Waihora is a broad, shallow coastal lake or waituna, in the 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, narrow, sandy K ...

. State Highway 75 to

Akaroa Akaroa is a small town on Banks Peninsula in the Canterbury Region of the South Island of New Zealand, situated within a harbour of the same name. The name Akaroa is Kāi Tahu Māori for "Long Harbour", which would be spelled in standard ...

and the Little River Rail Trail run along the north-western side of the lake. The lake is fed by the

Takiritawai River The Takiritawai River is a river of the Canterbury region of New Zealand's South Island. A short river of approximately , it flows from the confluence of the Okana and Okuti Rivers to feed the top of Lake Forsyth, at the southwest of Banks Pen ...

. Its natural discharge into the sea is through a gravel bank at the small community of

Birdlings Flat Birdlings Flat, originally named Te Mata Hapuku, is a settlement in Canterbury, New Zealand, at the eastern end of Kaitorete Spit and the southern end of Lake Forsyth, where the lake discharges to the sea. It is not far from eastern end of Lake ...

. Wairewa was an important source of eels as food for the Ngāi Tahu tribe (indigenous Maori people of the South Island). It is the only Ngāi Tahu customary lake. The Wairewa Rūnanga, one of 18 rūnanga of Ngāi Tahu, are the guardians or kaitiaki of the lake. Deforestation of the surrounding hills has led to erosion and silting up of the lake. The lake is hypertrophic, leading to eutrophication with corresponding poor water quality. This decline in water quality has been known since the early 1900s. In 2016, after a spell of dry weather, the water quality deteriorated and recurring algal blooms made the water toxic. Animals, including pets and sheep, died after drinking the water. The Wairewa Rūnanga has created an outlet canal that is occasionally opened to the sea when there is a risk of flooding due to heavy rain. The lake's health has improved since the canal was created. A bridge over the canal was built in 2018. Prior to that, vehicle access across the outlet was restricted for up to weeks at a time whenever the outlet canal was opened to the sea.

Geography

Lake Forsyth is long, narrow and shallow. It has undergone dynamic change over its short lifespan, from bay to estuary to lagoon. Up until a few thousand years ago it was an embayment in the Canterbury Bight, fully exposed to the fury of southerly waves. Over time, a spit of sand and gravel grew, fed by a strong

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

. The spit, grew steadily, until it ran into

and could grow no more, but the sand and sediment kept coming. The spit had become a barrier, now named

Kaitorete Spit Kaitorete Spit is a long finger of land which extends along the coast of Canterbury in the South Island of New Zealand. It runs west from Banks Peninsula for 25 kilometres, and separates the shallow Lake Ellesmere / Te Waihora from the Pacific O ...

. It closed off a nearby lake basin from the sea, creating the waituna now known as

. Within perhaps a further 1000 years, the barrier barred the mouth of the bay turning it into an estuary, where tidal waters could still go in and out. However, the progressive thickening of barrier impounded the estuary, kept the sea out, and it became a lagoon. Freshwater seeped out but sea water couldn’t flow it. The opening was navigable by waka canoes and small coastal schooners in the 1860s. If natural coastal processes were left to operate, Lake Forsyth would eventually become a lake. It is one of many lagoons and estuaries misnamed on New Zealand maps.

References

External links

{{Commons category, Lake Forsyth
Wairewa Research Project
– a collaborative project by the Wairewa runanga focussing on rehabilitation of Te Roto o Wairewa/Lake Forsyth Forsyth Geography of Christchurch Lagoons of New Zealand