Awaituna is a freshwater coastallagoon on a mixed sand and gravel (MSG) beach, formed where abraided river meets a coastline affected bylongshore drift.[1][2] This type of waterbody is neither a true lake, lagoon nor estuary.[2]
This classification differentiates it fromhapua, a type of river mouth lagoon. Both waituna and hapua are rare globally but common inNew Zealand, where they are considered ecologically significant as sites of traditionalMāori food-gathering as well as for their diversity of fish and bird species.[3]
Waituna form an interlinked chain of habitats which run the length of the east coast of theSouth Island: fromWairua Lagoon andLake Grassmere / Kapara Te Hau inMarlborough, throughLake Ellesmere / Te Waihora andCoopers Lagoon / Muriwai in centralCanterbury, andWashdyke Lagoon andWainono Lagoon in South Canterbury, toWaituna Lagoon inSouthland.[1]Waituna Lagoon is one of the finest examples.[4][5]
The name translates as “water of eels”.
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 areephemeral 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.[3]
Waituna are low in salt, high in water that lies around for a long time, and shallow – typically less than 3 metres (9.8 ft) 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.
Te Waihora / Lake Ellesmere is an outstanding natural feature. It is a wetland of international importance and an area of immense cultural significance toNgāi Tahu.[6]
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The waituna is fed by two small rivers,Selwyn River / Waikirikiri andIrwell River. These bring it a, comparatively, low volume of water, along with loads of silt and sand.
At the coast, in an area known asCanterbury Bight, a current runs parallel to the shore from south to north. This strong current brings with it sand and gravel sourced from theRakaia River. 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, aspit 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 intoBanks Peninsula. This stopped it growing, but the sediment kept coming, so it no longer fits the rules of a spit. It's now wider at theproximal end and narrower at thedistal end. It's become a barrier. Approximately 5000 years ago, it swung around, hinged at the point near Wairewamarae on the shores ofLake Forsyth / Waiwera.
The barrier, namedKaitorete Spit, 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 4 metres (13 ft) abovesea level.
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