Lake Rotoehu is located on SH 30 between Lake Rotoiti and Lake Rotoma. The lake has a total area of 800 ha and a catchment of 4710 ha. The catchment is around a third forestry, pastoral farming and native bush. The average depth of the lake is 8 metres and the deepest point of the lake is 13 metres.
Lake was formed around 8500 years ago and it has no surface outlet. The discharge from the lake is via groundwater - flowing towards a northerly direction. The historical meaning of Rotoehu, "turbid or murky" water, implies that the lake may never have been clear. The water quality remained fairly stable until 1993 when the level of nutrients and algae rose dramatically. This was attributed to a drop in the water level and a warm summer. It is a shallow lake with geothermal inputs, and nutrient levels remain high.
In the 1960s, lake researchers noted that the algal production in the lake was occasionally sufficient to cause algal blooms to develop . This is an indication that Lake Rotoehu was nutrient enriched to probably a mesotrophic state about this time. Water clarity was reduced by about one metre, and the oxygen content in the bottom waters dropped to low levels in summer, into the 1970s . This water quality change reflected the land use changes in the catchment over these decades from native bush and scrub to pasture.
The small communities around Otautu Bay, Kennedy Bay and the rest of the community have a very strong affinity with the lake. Most lake use is by local residents, particularly boating and fishing. These communities are saddened by the algal blooms and hornwort infestations that have plagued the lake since 1993. While local residents are willing to help with restoration of the lake, they believe Environment Bay of Plenty needs to focus and direct efforts to fix up their lake and return it to pre-1993 or better water quality.
Water quality in Lake Rotoehu sitting above its target TLI. While actions over the last few years including phosphorus locking and weed harvesting have been taken there is still more work to be done.