Lake Grassmere / Kapara Te Hau is a shallow coastal lagoon located in the Marlborough Region of New Zealand's northeastern South Island, approximately 40 kilometres south of Blenheim and near Clifford Bay close to Cook Strait. Covering around 17 to 18 square kilometres with a mean depth of roughly one metre and a shoreline perimeter of approximately 43 kilometres, it is separated from the Pacific Ocean by a narrow barrier beach of sand, gravel, and dunes stretching about 20 kilometres.

Paddlers should be aware that the lake has been extensively modified for commercial solar salt production and is not a natural open water body in the conventional sense. The lake area is subdivided into a series of managed evaporation ponds with gates controlling water flow, and seawater is actively pumped in from the adjacent ocean. This industrial use fundamentally shapes the character of the water body and likely restricts or precludes recreational paddling across much of its area.

The lake has no natural river inflows and is subject to strong, warm drying winds — conditions that can make open-water paddling challenging. Salinity levels are extremely high, well above that of normal seawater, as the entire operation depends on evaporative concentration. During summer months, the evaporation ponds take on a distinctive pink colouration due to salt-tolerant algae, and brine shrimps are also present at certain salinity levels.

The site sits in a tectonically active zone at the transition between the Marlborough Fault System and the Hikurangi Subduction Zone. A magnitude 6.6 earthquake occurred directly beneath the lake in August 2013 at a depth of 7 kilometres, a reminder of the region's seismic character.

The lake carries significant Māori cultural history. Known as Kāpara Te Hau — meaning "wind-blown lake" — it features in traditions of Rangitāne, Ngāti Kuia, and Ngāi Tahu, and was the site of a notable battle in the early 1830s involving Te Rauparaha. Given the industrial nature of the site and the absence of documented paddling access or routes, those considering a visit should confirm current access permissions with the salt works operator before planning any on-water activity.