Lake Rotomahana
Template:Short description Template:Use New Zealand English
Template:Infobox body of water Lake Rotomahana is an Template:Convert<ref name=White2016/>Template:Rp 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 Template:Lang, following Hochstetter.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite book</ref> and the surrounds of the lake had become world famous following its first European written description in 1843.<ref>Template:Cite book</ref>
Geology
Lake Rotomahana is one of the most studied lakes in New Zealand, occupying the southwestern portion of a Template:Convert 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 Template:Convert,<ref name=Ronde2018>Template:Cite journal</ref> not the geographically mapped height of Template:Convert,<ref name="LINZ">LINZ Topo50 Map BF37</ref> or the mean height of Template:Convert.<ref name=White2016/>Template:Rp Lake Rotomahana has no natural surface outlet, and its water level varies by about one metre in response to rainfall and evaporation.<ref>Information panel at Lake Rotomahana's shore</ref> There is now an engineered surface channel to maintain maximum lake level.<ref name=White2016/>Template:Rp 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 Template:Convert.<ref name=Gillon2009>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref>Template:Rp<ref name=White2016>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>Template:Rp
Before the 1886 eruption, two small lakes were present in the current lake's basin and perhaps six smaller ponds.<ref name=Bunn2023>Template:Cite journal</ref>Template:Rp The other lake was called Lake Rotomakariri.<ref name="Thomas1888">Template:Cite book</ref> After the eruption a new Lake Rotomakariri to its east briefly existed before being incorporated by lake level rise into the present lake.<ref name=Bunn2023/> The now lake floor was mostly dry when surveyed in the period 1886 to 1888 after the eruption,<ref name="Thomas1888M">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and it is possible to match these observations with current geolocated hydrophonic findings.<ref name=Ronde2018/>Template:Rp High quality pictures of the old Lake Rotomahana and associated tourist attractions were widely available in Europe by 1875.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 Template:Convert deep,<ref name=Ronde2018/>Template:Rp the deepest in the Rotorua district. The former official depth was previously Template:Convert,<ref name="lowe" /> at a lower lake level and with less precise equipment.<ref name=Ronde2018/>Template:Rp The lake bottom currently has up to Template:Convert of sediments, which means the Rotomahana crater bottom is Template:Convert above sea level.<ref name=Ronde2018/>Template:Rp
The lake's northern shore lies close to the Template:Convert lower Lake Tarawera,<ref name=Hodgson2005>Template:Cite journal</ref> separated by less than Template:Convert 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.<ref name=Ronde2018/>Template:Rp<ref name=Bunn2023/>Template:Rp
The consensus range of difference with current water level between the old and current Lake Rotomahana appears to be Template:Convert.<ref name=Bunn2023/><ref name=Ronde2018/>Template:Rp Template:Efn
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.<ref name="Lowe2023">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The eruption process which was basaltic deposited muddy material widely and many metres thick especially to the northeast.<ref>Template:Cite book</ref> 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.<ref name="Thomas1888"/>Template:Rp Essentially the new crater evacuated material to a depth of at least Template:Convert where the pre-eruption lakes had been and possibly as much as Template:Convert in places.<ref name=Ronde2018/>Template:Rp
The current average conductive heat flux is at least three times higher than that either beneath Lake Rotorua or Lake Taupō.<ref name=Whiteford1994>Template:Cite journal</ref> The latest determination is 47 MW.<ref name=Tivey2016>Template:Cite journal</ref> 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.<ref name=Tivey2016/> 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.<ref name=Tivey2016/>
_(1873)_(14782100062).jpg)
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.<ref>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref> 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.<ref name="Bunn R 2020">Template:Cite journal</ref> 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,<ref name=White2016/>Template:Rp but has high trophic level index inflow from the Okaro catchment via the Haumi Stream.<ref name=White2016/>Template:Rp
Water Inflows
| Source | Details | Mean inflow to lake |
|---|---|---|
| Haumi Stream | At lake | Template:Convert |
| - | Haumi Stream above Watmangu Stream confluence | Template:Convert |
| - | Watmangu Stream | Template:Convert |
| Te Kauae Stream | Ash Pit Rd Ford | Template:Convert |
| Putunoa Stream | Farm Track Culvert | Template:Convert |
| Rotomahana Stream | at swamp | Template:Convert |
| Catchment Inflow | from precipitation | Template:Convert |
| Catchment Inflow | estimated subsurface other catchments | Template:Convert |
Pink and White Terraces
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:Annotated image The Pink and White Terraces were a natural wonder on the shores of the lake before the 1886 eruption. They were considered to be the eighth wonder of the natural world and were New Zealand's most famous tourist attraction during the 19th century, from c. 1870-1886; but were buried or destroyed by the eruption.<ref>Template:Cite news</ref>
Scientists thought they had rediscovered the lower tiers of the Pink and White Terraces on the lake bed at a depth of Template:Convert in 2011.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> More recent research reports over 2016-2020 suggest the upper parts of both terraces lie on land and may therefore be accessed for physical evidence the terraces or sections of them survived in their original locations.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Bunn R 2020"/>
The 2017- research relied on the journals of German-Austrian geologist Ferdinand von Hochstetter, who visited the lake in 1859.<ref>Template:Cite news</ref> Hochstetter's journals are the only known survey of the terraces before the eruption.<ref name=":0">Template:Cite news</ref> Using Hochstetter's field diaries and compass data, a team of New Zealand researchers identified a location where they believe the Pink and White Terraces lie preserved at a depth of 10–15 metres (32–49 ft).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The researchers were hoping to raise funds for a full survey of the area, but any work would first have to be approved by the local Māori tribe on whose sacred ancestral land the Pink and White Terraces are situated.<ref>Template:Cite news</ref><ref name=":0" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Ground penetrating radar searches were undertaken in 2017 but the equipment failed to penetrate sufficiently deeply to show whether or not the terraces lay in their surveyed locations.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Later Hochstetter survey research refined the Pink, Black and White Terrace locations.<ref name=S2018>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Bunn, A. R. (2019). Hochstetter’s survey of the Pink and White Terraces: the final iteration. Surveying+Spatial 99, 30–35.</ref> The issue of whether any of the terraces remain continues to remain unresolved.<ref name=Bunn2023/>
Green Lake
A small lake, Green Lake, lies close to the eastern shore of Lake Rotomahana at Template:Coord. It should not be confused by the much larger Lake Rotokākahi (Green Lake), which lies to the west of Rotomahana.
Green Lake was formed in a roughly circular crater and is some 100 metres in diameter. It takes its name from its distinctive colour, which is considerably greener and darker than that of Rotomahana. The lake formed after the 1886 eruption of Mount Tarawera. Prior to the eruption, a small (~12m diameter) lakelet also known as Green Lake (Lake Rotopounamu) had existed to the north of Lake Rotomahana in Waikanapanapa Valley but this was exhumed during the Tarawera eruption. After the eruption, water flowed into the new Green Lake crater, which was given the same name as the older lake.<ref>Cole, J. W., (1970) "Structure and eruptive history of the Tarawera volcanic complex", New Zealand Journal of Geology and Geophysics 13, 4. pp. 879–902.</ref> Other pre-eruption lakes and lakelets about Lake Rotomahana included Lakes Rotomakariri, Rangipakaru, Ruahoata and Wairake. The shape, location and orientation of Lake Makariri in Cole, 1970 (cited herein) is incorrect. He followed August Petermann's flawed map. Hochstetter shows the lake axis lay at an azimuth of 355 degrees.<ref>Template:Cite book</ref> Recent research into these lake levels gave insight into changes at Lake Rotomahana in the lead-up to the eruption.<ref name=S2018/>
The Sunken Totara Forest Myth
One forgotten lake feature is the semi-mythical sunken totara forest of Lake Rotomahana.<ref>Template:Cite book</ref> In 2016 when a scuba team first dove the lake, they found no evidence of a sunken forest or trees, as reported by Fitzgerald off Moura.<ref>Template:Cite book</ref><ref>Bunn, A. R. Quest for the Pink and White Terraces, rushes, 7.30-8.30, https://www.youtube.com/watch?v=L9YHJjPTrX8 , uploaded 6/5/2016.</ref> While there were forests over the pre-eruption Mt Tarawera, Tōtara trees were scattered and only recorded over the western and southern mountain flanks.<ref>Template:Cite journal</ref> Given Tōtara tree groves could hardly appear in the eruption craters, it appears likely any sunken forest lies in the north-east corner of the new lake.
The placement of the sunken forest would have been post-eruption, via the mechanism described by the US Forest Service after the Mt. St. Helens eruption.<ref>Walker, T. (2017) “Learning the Lessons of Mount St Helens”, Creation 39, 3, 26</ref> The trees were uprooted in the eruption and propelled into the crater during or after the eruption. As the new lake formed over decades, the trunks floated for a time, then tipped vertically; later descending into a vertical lie, and became embedded into the lake floor coming to resemble a sunken forest. Hence, another myth about the Tarawera eruption and Lake Rotomahana is explained.