Lake Rotomahana is an 890-hectare (2,200-acre) lake in northern New Zealand, located 20 kilometres to the south-east of Rotorua. It is immediately south-west of the dormant volcano Mount Tarawera, and its geography was substantially altered by a major 1886 eruption of Mount Tarawera. Along with the mountain, it lies within the Ōkataina Caldera. It is the most recently formed larger natural lake in New Zealand, and the deepest in the Rotorua district.
History The New Zealand Ministry for Culture and Heritage gives a translation of "warm lake" for Rotomahana, following Hochstetter. and the surrounds of the lake had become world famous following its first European written description in 1843.
Geology Lake Rotomahana is one of the most studied lakes in New Zealand, occupying the southwestern portion of a 17-kilometre-long (11 mi) rift which formed during the 1886 eruption of Mount Tarawera. In the context of the geological discussion, a high standard hydrophonic survey took place in 2016 and the absolute measurements taken at this time, as lake level varies, are used for baseline. At this time the lake level was 340 m (1,120 ft), not the geographically mapped height of 337 m (1,106 ft), or the mean height of 338.7 m (1,111 ft). Lake Rotomahana has no natural surface outlet, and its water level varies by about one metre in response to rainfall and evaporation. There is now an engineered surface channel to maintain maximum lake level. Hydrogeologic models of the catchments in the Ōkataina Caldera predict that the Lake Rotomahana catchment is contributing subsurface to the Lake Tarawera catchment with an outflow, depending upon the size of the catchments so defined, of between 1,125–3,018 L/s (39.7–106.6 cu ft/s). Before the 1886 eruption, two small lakes were present in the current lake's basin and perhaps six smaller ponds. The other lake was called Lake Rotomakariri. After the eruption a new Lake Rotomakariri to its east briefly existed before being incorporated by lake level rise into the present lake. The now lake floor was mostly dry when surveyed in the period 1886 to 1888 after the eruption, and it is possible to match these observations with current geolocated hydrophonic findings. High quality pictures of the old Lake Rotomahana and associated tourist attractions were widely available in Europe by 1875. Following the eruption, a number of craters filled over the course of 15 years to form today's Lake Rotomahana. As a result it is the most recently formed large natural lake in New Zealand, and at 118 m (387 ft) deep, the deepest in the Rotorua district. The former official depth was previously 112.4 m (369 ft), at a lower lake level and with less precise equipment. The lake bottom currently has up to 37 m (121 ft) of sediments, which means the Rotomahana crater bottom is 185 m (607 ft) above sea level. The lake's northern shore lies close to the 39-metre (128 ft) lower Lake Tarawera, separated by less than 700 metres (2,300 ft) of terrain that is mostly material from the 1886 eruption. The original Lake Rotomahana has a slightly controversial level with respect to that of Lake Tarawera before 1886, partially because the eruption also changed the level of Lake Tarawera and there was a later lowering of Lake Tarawera's level around 1904. The consensus range of difference with current water level between the old and current Lake Rotomahana appears to be 35–48 m (115–157 ft). The original lake formed in an area of mostly rhyolytic eruptives and would have also been associated with lake sediments deposited at least in the time since the 1314 ± 12 CE Kaharoa eruption of Mount Tarawera. The eruption process which was basaltic deposited muddy material widely and many metres thick especially to the northeast. One or other of the lakes that existed before the eruption is the likely origin of shells of water-snails found, in a sample of fresh ash from Tauranga, and in an ash sample from Cape Runaway. Essentially the new crater evacuated material to a depth of at least 60 m (200 ft) where the pre-eruption lakes had been and possibly as much as 80 m (260 ft) in places. The current average conductive heat flux is at least three times higher than that either beneath Lake Rotorua or Lake Taupō. The latest determination is 47 MW. The hydrothermal system that feeds still active geothermal features on the lake shore and had fed the Pink Terraces at the western side of the lake, has a heat flux of 21.3 W/m2. There is an area southwest of Pātītī Island that has a heat flux averaging 13 W/m2 that appears to be in a lake floor crater created in 1886.
Ecology The lake is a wildlife refuge (and was one prior to first contact), with all hunting of birds prohibited. A healthy population of black swan inhabits the lake, and there are efforts underway to ensure the lake's largest island, Pātītī Island, is kept pest-free. Recent research confirms Pātītī Island is the closest surviving pre-eruption feature on the old lake, i.e. to survive the 1886 eruption, being formerly known as Rangipakaru Hill. There is no public access to the lake, save for the Tourist Track, overland from Lake Tarawera. A boat cruise on the lake, visiting hydrothermal features on the lake's shore, is available as an additional extra from the Waimangu Volcanic Valley tourism operation. The nitrogen load on the lake is stable, but has high trophic level index inflow from the Okaro catchment via the Haumi Stream.
Water Inflows
Pink and White Terraces
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