Last Modified: 15-7-2019 18:26
By: Christopher Cookson
Lake Grassmere is a shallow lake near Ward that is used for salt production, and takes its name from the Weld homestead. In Rangitāne tradition, the lake is known as kāpara-te-hau and formed after a battle between Kupe and Rangitāne ancestor Te Hau, who had planted kumara gardens on what is now the lake. According to tradition, Kupe sent a tsumani that inundated the land, drowining Te Hau's garden forming the lake. Geological records indicate that Lake Grassmere formed about 1800 years ago in the Ward depression as a shallow bay only to become enclosed by gravel moving northwards along the coast. In a modern parallel to Rangitāne tradition, a strong magnitude 6.6 earthquake took place under the lake at a depth of 7km, on August 16, 2013, causing damage to nearby townships of Seddon and Ward.
The 1750ha lake has no rivers flowing into it, a rainfall of around 600mm per year and hot drying winds. This dries out the lake and increases the salinity, making Lake Grassmere ideal for natural salt production. Before being modified it attracted a wide range of water birds and likewise human hunters. In one such hunting excursion Te Rauparaha, having just massacred large numbers of South Island Māori, sought to feed his men by catching moulting ducks. Ngāi Tahu warrior Tuhawaihi was alerted to this and came well prepared, ambushing Te Rauparaha and his men and taking him captive. This event almost spelt the end of Te Rauparaha, however he managed to slip his ropes during the night, although many other members of his party were killed.
Lake Grassmere is subdivided into a series of ponds with gates to allow water in and out. Water is pumped from the sea into the first of the ponds. From there it moves form pond to pond over several months until when at the last one the salinity is at a level that salt crystals are formed. These are harvested and processed. Economic byproducts include gypsum and caustic soda.
During the evaporation process, the high salinity leads to proliferation of algae which can tolerate the harsh conditions, giving the evaporation ponds their distinctive pink colour over the summer months before harvest. Before the salinity reaches too high a level, brine shrimps also thrive in the evaporation ponds.