Selwyn River, New Zealand
Encyclopedia
The Selwyn River / Waikirikiri flows through the Selwyn District
of Canterbury
in the South Island
of New Zealand
.
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 disappear beneath this bed and seem to dry up completely. This frequently occurs where State Highway 1
crosses the river at the settlement of Selwyn, about 20 kilometres upstream from its outflow into Lake 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 remains 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.
Selwyn District
The Selwyn District is a predominantly rural area in central Canterbury, on the east coast of New Zealand's South Island. It is named after the Selwyn River, which is in turn named after Bishop George Selwyn, the first Anglican bishop of New Zealand who, in 1843 and 1844, travelled the length of...
of Canterbury
Canterbury, New Zealand
The New Zealand region of Canterbury is mainly composed of the Canterbury Plains and the surrounding mountains. Its main city, Christchurch, hosts the main office of the Christchurch City Council, the Canterbury Regional Council - called Environment Canterbury - and the University of Canterbury.-...
in the South Island
South Island
The South Island is the larger of the two major islands of New Zealand, the other being the more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman Sea, to the south and east by the Pacific Ocean...
of New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
.
The river has its source in the foothills of the Southern Alps
Southern Alps
The Southern Alps is a mountain range extending along much of the length of New Zealand's South Island, reaching its greatest elevations near the island's western side...
and flows east for 80 kilometres before emptying into Lake Ellesmere
Lake Ellesmere
Lake Ellesmere / Te Waihora is located in the Canterbury Region of the South Island of New Zealand. It is actually a broad, shallow lagoon located directly to the west of Banks Peninsula, separated from the Pacific Ocean by a long narrow sandy spit called Kaitorete Spit, or more correctly Kaitorete...
, to the south of Banks Peninsula
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...
. Terrace cliffs above the river's upper reaches gave the town of Whitecliffs
Whitecliffs
Whitecliffs is a small town with a population of 117 in the Selwyn District of the Canterbury region of New Zealand's South Island. It has also been known as South Malvern , and the name of Whitecliffs comes from terrace cliffs above the Selwyn River...
its name.
For much of its course the river flows through wide shingle channels. In drought years, the river can disappear beneath this bed and seem to dry up completely. This frequently occurs where State Highway 1
New Zealand State Highway network
The New Zealand State Highway network is the major national highway network in New Zealand. Just under 100 roads in both the North and South Islands are State Highways...
crosses the river at the settlement of Selwyn, about 20 kilometres upstream from its outflow into Lake 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
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
. 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 remains dry for most of the year, apart from a small section around the confluence with the permanently flowing Hororata River
Hororata River
The Hororata River is a river of Canterbury, in the South Island of New Zealand. A tributary of the Selwyn River, the Hororata has its sources in rough hill country north of Windwhistle, and flows east through the township of Hororata before reaching the Selwyn 12 kilomewtres northwest of...
. 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.