Selwyn River

The Selwyn River/Waikirikiri is located in the Selwyn District of Canterbury in the South Island of New Zealand. [ [http://www.linz.govt.nz/apps/placenames/index.html?p=125774 Selwyn River/Waikirikiri] , New Zealand Geographic Placenames Database, Place Name Detail, Land Information New Zealand web site, retrieved 5 April 2008.]

The river has its source in the foothills of the Southern Alps and flows east for 80 kilometres before emptying into Lake Ellesmere, to the south of Banks Peninsula. Terrace cliffs above the river's upper reaches gave the town of Whitecliffs its name.

For much of its course the river flows through wide shingle channels. In drought years, the river can be lost beneath this bed, and appear to dry up completely. This can happen as close to its outflow as where it is crossed by State Highway 1, at the settlement of Selwyn, only 20 kilometres from Ellesmere.

In the foothills, the Selwyn flows year-round. On the plains, the riverbed is highly permeable, and the river overlays a deep and porous aquifer. As soon as the river reaches the plains, water begins leaking down through the bed and into the aquifer. In most months, all river water disappears within 5 km of leaving the foothills. The next 35 km of the river is dry for most of the year, apart from a small section around the confluence with the permanently flowing Hororata River. About 15 km upstream from Lake Ellesmere, shallow groundwater rises back to the surface, and the Selwyn becomes permanent again.

Disappearing river flows have significant ecological effects: when the river’s surface water disappears, so does the habitat for many aquatic plants and animals. In response to loss of surface water, aquatic invertebrates and fish must disperse, seek refuge in remnant aquatic habitats, or die. Aquatic plants, algae, and bacteria must form resting stages, or die. The dry central reaches of the Selwyn River also constitute a significant barrier for dispersal of invertebrates and for fish migrating between Lake Ellesmere and the headwaters. [ [http://www.niwa.cri.nz/news/mr/2004/?a=27394 Biodiversity in a disappearing river] , Mike Scarsbrook, Scott Larned, Graham Fenwick, and Dave Kelly,]

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