Te Waihora/Lake Ellesmere is the largest lake in Canterbury and has no natural outlet to the sea. It is of outstanding cultural significance to Ngāi Tahu. The lake is a tribal taonga, reflecting its importance to mana whenua from times when it was a major tribal resource. It is also the most important wetland habitat of its type in New Zealand, with a high, diverse bird population and its support to many native fish species.
The lake was opened by generations of Ngāi Tahu before Pākehā arrival. The first written settler's record of an artificial opening between the lake and sea was in 1852. It has been opened more than 300 times since.
There is a lot of interest surrounding the opening process and reasoning. Below we have answered a few of the most common questions.
Questions and Answers
Why do we need to open the lake?
Opening the lake is required because if left in its natural state, the water level would rise to around 4m before naturally breaching the Kaitorete Barrier. The corresponding water surface area would be very large, causing widespread flooding in the Selwyn District and Christchurch city areas. The lake is also opened for fish migration in and out of the lake and to protect or enhance other environmental or cultural values.
Once the lake is opened, there is limited ability to control how long it stays that way. The length of time it stays open is determined by weather and sea conditions, and how quickly gravel is deposited by the sea to close the cut.
The option of constructing a permanent opening has been considered but is not logistically possible due to the surrounding environment. The various options that have been considered over the years are documented here.
Who decides if and when to open the lake?
Te Waihora/Lake Ellesmere's opening is governed by a National Water Conservation Order and a range of resource consents held jointly by Te Rūnanga o Ngāi Tahu and Environment Canterbury.
There is a minimum level the lake must reach before it may be opened; from 1 April to 31 July the minimum level is 1.13m, and from 1 August to 31 May the minimum level is 1.05m. There must then be an agreement between joint consent partners, Environment Canterbury and Ngāi Tahu, to open the lake. Most importantly, the conditions must be safe enough to enable people and machinery to perform the opening without any risk to their safety. The maximum level of sea swell in which a lake opening can safely occur is up to 2m.
Many groups have an opportunity to voice their views before a final joint decision is made. You can find out more about this here.
What is the process around opening the lake in emergency situations such as the recent flooding in July 2017?
The lake is opened for a variety of reasons including managing land inundation. During a flooding event in July 2017, it was not as simple as being just a high lake level – significant flows in the Selwyn River/Waikirikiri and its tributaries, Halswell/Hurutini and widespread surface water flooding led to extensive inundation of the Selwyn District as well as other parts of Canterbury. Parts of the lower Selwyn River catchment were flooded by overtopping stopbanks and surface water rather than lake levels.
The maximum level of swell in which a lake opening can safely occur is up to 2m. During the July 2017 weather events, swells were forecast to be up to 7m at the same time the rain was falling and therefore a pre-emptive lake opening could not take place. Swell levels dropped enough for work to safely begin two days after the rain had ceased. It took 24hrs to open the lake once work started on this occasion.
What makes it so unsafe to open when the sea is high?
To open the lake, machinery needs to work in the surf zone at the last part of the process. Waves larger than 2m could swamp machinery.
Can you at least make a start when the sea is high?
Ahead of an opening parts of the works can occur ahead of time if conditions allow – primarily marking out the cut and any in-lake work need to deepen water or maintain the seawall/gravel bund.
What does it cost to open the lake?
Openings can cost anywhere from $20,000 to $120,000, depending on the earthworks needed on-site. The lake is opened to the sea between 2-6 times a year.
Who pays for the openings?
Openings are funded and managed through the Lake Ellesmere Rating District. The funding split is a 50 per cent targeted rate, 25 per cent works and services rate and 25 per cent general rate.
When do you close the lake?
The lake can be closed at certain times of the year if it falls below 0.60m. To date, it has never been attempted. The volume of material needed to try and fill in the cut and the right sea conditions are the main limitations.
What does opening the lake do for fishing in the area?
Where possible we try to align openings with fish migration periods and other habitat values of the lake. Tuna (freshwater eels) come in from April to June via kōumu/drains or a full opening. Pātiki/flounder, inānga/whitebait, sea-run trout and other species enter the lake around September and October.
Recreational fishing in Te Waihora/Lake Ellesmere and its tributaries includes the fishing of brown trout, flounder, tuna (eel), coarse fish species such as perch, and whitebaiting when the lake is open to the sea. Good numbers of sea-run trout are present when the lake is open to the sea and brown trout fishing can take place in the lower reaches and at the mouths of inflowing streams.
Te Waihora/Lake Ellesmere is one of New Zealand’s most polluted lakes. What is being done to address this?
Te Waihora/Lake Ellesmere was once the pride of Ngāi Tahu and a considerable tribal resource. Today, it is one of New Zealand’s most polluted lakes. Decades of settlement and farming have taken their toll and although it is far from ‘dead’, the lake has been significantly degraded.
Today, there is widespread acceptance of the need to improve water across all of Selwyn, and the Selwyn Waihora Zone Committee, Environment Canterbury, Ngāi Tahu and many other organisations are working hard with farmers and the local community to implement solutions. Selwyn Te Waihora - Our Water Story outlines the work that has been done in the catchment since the Canterbury Water Management Strategy was introduced.