Lake Rotorua is central to the Rotorua Lakes district and is the main lake of the area. The city of Rotorua is on the southern shore of this Lake. Lake Rotorua is a relatively shallow lake that was formed approximately 140 000 years ago. Hot springs provide direct geothermal inputs along the southern shore (EBOP, 2000). The Ohau channel is the only outflow from the lake, which links Lake Rotorua with Lake Rotoiti. There are nine major inflow streams and eighteen minor inflow streams. The total area of the lake is 8060 Ha and the catchment area is 50,060 Ha. The lake has an average depth of 11 m and at its deepest point it is 45 m.
The Lake Rotorua catchment is the most densely populated of all the Rotorua Lakes, with 8.1% of the catchment urban (EBOP, 2000).
A number of the Rotorua lakes were named by Ihenga, a grandson of the captain of the Arawa canoe Tamatekapua. He named the largest lake, Rotorua nui ā Kahu-matamomoe, in honour of his father-in-law and uncle, Kahumatamomoe. "Rotorua nui" refers to the large basin-like lake. Lake Rotorua probably has New Zealand's best-known lake islands, and is closely associated with one of the best-known Māori legends, that of Hinemoa and Tutanekai. It is said that Hinemoa swam across the lake to her lover Tutanekai who lived on Mokoia Island.
Great achievements have been made for Lake Rotorua and the water quality continues to be the best it has been in decades. The lake remains at or around its target Trophic Level Index (TLI) of 4.2 depending on the climatic conditions. Regional Council doses aluminium sulphate into two streams which flow to the lake, the Puarenga and Utuhina, the aluminium sulphate locks the phosphorous into the sediment removing it from the water column. This intervention is a key reason why the lake is achieving its water quality target but this is not a long term solution for the lake.
An integrated framework of solutions aims to achieve long term water quality. It includes:
- Proposed Plan Change 10 – a set of rules to remove 140 tonne of nitrogen from entering Lake Rotorua from pastoral farming.
- Gorse Conversion Programme - $2.5 million fund to remove 30 tonnes of nitrogen from entering Lake Rotorua.
- Lake Rotorua Incentives Programme - $40 million incentive fund to remove 100 tonnes of nitrogen from entering Lake Rotorua through voluntary land use/management change.
- Engineering solutions – Bay of Plenty Regional Council will use engineering initiatives to remove 50 tonnes of nitrogen from entering Lake Rotorua.