The lake basin originated in a caldera about 100,000 years ago. Further volcanic activity changed the shape and drainage from the basin. The present lake size and outflow were established about 13,000 years ago but geothermal activity is still evident near the lake which receives hot water inflows from several springs and geysers near its southern shore, some of which carry white colloidal silica.

The lake has become highly eutrophic in recent years. The source of about half its N and P income is from the inflow of effluent from sewage derived from Rotorua City and satellite villages situated along its shores. In addition, its modest native flora of aquatic plants has been progressively displaced by successive invasions of exotic species; Elodea canadensis in the 1950's, Lagarosiphon major in the 1960's and Ceratophyllum demersum in the 1970's. These three constitute the extensive stands of lakeweed that now infest the shallow, inshore region of the lake. The authorities have to solve both these problems in order to maintain the important recreational and tourist values of Lake Rotorua.

Some reduction in the lake nutrient income has been effected by the Upper Kaituna Catchment Control Scheme under which, from 1980, the inflows of the lake were surveyed and measures taken to stabilize the river banks for erosion control and the leaching of nutrients from pastures. Plans are also well advanced for major modification of the city sewage treatment and effluent disposal methods which will prevent this large source of nutrients from entering the lake. This project should become operational by 1990.

There is no feasible solution to the problem of lakeweed infestation but the Department of Conservation in Rotorua maintains a carefully monitored programme of herbicide spraying which keeps navigational hazards to a minimum and reduces excessively heavy stranding of plant material along the lake shores after heavy storms (Q, 1).