Opening Te Waihora/Lake Ellesmere

Te Waihora/Lake Ellesmere is the largest lake in Canterbury and has no natural outlet to the sea. It is valued for cultural and environmental reasons and 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 over 300 times since.

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. However, many groups have an opportunity to air their views before a final decision is made.

Openings tend to be aligned 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.

Factors to take into account include:

• access for traditional mahinga kai including gathering of swan's eggs
• avoiding low lake levels in summer and desiccation of wetland margins
• the need for variation in lake levels to support complex and diverse wetland bird and plant habitats
• managing land inundation (including from wind effects)
• the effects of high lake levels on drain networks and infrastructure.

Experts provide technical advice around the machinery. The weather and sea conditions are forecast while lake levels are monitored. It generally takes three to seven days to forge the opening but if sea conditions are rough, past experience has shown it could take up to six weeks.

Historical methods for opening the lake have included horse-drawn scoops and two outlet culverts in the early part of the 20th century — Dobson’s Culvert and Pannett’s Culvert. Heavy machinery has been used since the 1930s. For more details, read these accounts.

Once the lake is opened, there is limited ability to control how long it stays that way. The length is determined by weather and sea conditions, and how quickly gravel is deposited by the sea to close the cut.

Artificial closure has not been attempted to date however the water order allows for the lake to be artificially closed when it reaches 0.6 metres above sea level between 1 October and 31 March. Such a closure would only be considered in summer if lake levels were very low and sea conditions were too calm to create natural closure.

We monitor effects of the lake opening including:

• keeping an eye on channels through the spit from the lake edge to assist tuna/freshwater eel migration
• sediment sampling and experimental dredging of the deeper channels of the lake bed to work out whether this will result in greater flushing to improve water quality and associated wildlife and mahinga kai values
• investigating the feasibility of opening the lake at the southern end of Te Korua near the mouth of the Waikekewai Creek and the conditions needed in the lake to make a successful opening in this area (Te Korua was the traditional opening site used by Ngāi Tahu)
• Investigating the feasibility of establishing a controlled outlet from Te Waihora/Lake Ellesmere to the sea.

• View reports on Te Waihora/Lake Ellesmere