Lake Grassmere, 25 miles south of Blenheim and 6 miles south of the mouth of the Awatere River, is a shallow lagoon protected from the open sea by a single barrier beach covered by sand dunes. The lagoon lies on the north-easterly extension of the Ward depression. Its area varies between 3,500 and 4,400 acres; this maximum is attained only in rare floods. The watershed is small and there are no large streams draining into it. The climate, with a low average rainfall of 24 in. and prevailing strong and dry north-westerly winds, provides Lake Grassmere with the suitable conditions required for natural economic salt production.
This industry began in 1943 and now occupies one-third of the suitable lake area of 3,800 acres. The 1960 production was 17,000 tons. The total consumption of industrial and domestic salt for New Zealand for that year was about 61,000 tons. Maximum expansion could produce 50,000 tons, and many economic by-products can be developed; caustic soda and gypsum are already produced in small quantities.
The salt harvest for the period 1963–64 was a record one, 24,000 tons being won. It was the first season in which the harvest was carried out on the new pattern of crystallising ponds which have special provision for draining off rainwater before it has had time to mix with the brine concentrate. The harvest for 1964–65 is expected to reach 30,000 tons — a new record.
Grassmere was the name of the homestead of F. A. Weld on the Flaxbourne Run, and was later applied to the lake. According to a legend, the lake occupies the site of cultivations known as Ka-para-te-hau (early whalers on the coast rendered this name “Cobblers' Hole”). Kupe, the navigator, is said to have poured salt water on these cultivations, thus creating the lake. Some time after his successful raid on Kaiapohia (Kaiapoi), Te Rauparaha visited the lake with a party to take moulting ducks. A Ngai Tahu force, having learned of this impending visit, laid an ambush. Scouts from Te Rauparaha's party found the locality apparently uninhabited and the canoes were driven ashore. On landing, the visitors were taken by surprise and sustained heavy casualties. Te Rauparaha was seized by Tuhawaiki, who was anxious to take him alive. The captive, however, after feigning resignation, was less tightly held. He then slipped out of his flax garment, swam to a canoe and made his escape.
by Geert Jan Lensen, New Zealand Geological Survey, Lower Hutt.