The park with its high rainfall forms the major water catchment area for much of the eastern Bay of Plenty and northern Hawke’s Bay and includes rivers, lakes, lakelets and tarns. The headwaters of the Whakatane and Waimana/Tauranga Rivers, drain to the north while tributaries of the Rangitaiki River, including the Horomanga Stream drain the western side of the park. The Waioeka River has its headwaters in the park, and drains to the east. To the south and east flow the Waiau, Waikaretaheke and Ruakituri Rivers which meet to form the Wairoa River with its outlet at Wairoa.
Lake Waikaremoana is the second largest lake in the North Island (5, 400ha) and the deepest at 248 metres. It is also one of the larger lakes in the North Island that is oligotrophic. The lake was formed approximately 2, 200 years ago by a landslide that created a massive rock and earth fill dam. The lake supports diverse, predominantly native aquatic plant communities and is a significant freshwater habitat. There are however three introduced aquatic plant species that are well established. Only one of these, Elodea canadensis, ever dominates the aquatic vegetation and then only locally, for example in Home Bay. In 1999 the first instance of transfer of the more vigorous aquatic weed (Lagarosiphon major) to Waikaremoana was detected in Rosie Bay. Control measures are being implemented.
Lake Waikaremoana provides important recreational opportunities including boating and fishing. The Great Walk around the lake is a significant recreational attraction and for many the lake is an icon for the park, attracting a large number of visitors.
Since 1929 the lake has provided water storage for hydroelectric power stations on the Waikaretaheike River – initially at Tuai only, then at Piripaua and Kaitawa as well. Until 1946 the lake outlet was not controlled and the lake level fluctuated naturally. When the lake level was about or below its average level, all outflow was underground through leaks in the lakebed, emerging as springs. In 1946, the lake level was lowered to increase the flow through the power station and reduce the natural leakage. The majority of the lakebed leaks have been sealed, and most outflow from the lake is now through the intake tunnel directly to the Kaitawa power station.
The lake is now operating at average levels five metres below the natural levels. Wavecut terraces formed by the natural lake and extensive areas of shallow littoral (nearshore) zone of the stream deltas are exposed as a result of the lowered lake levels. A new cycle of erosion and related shoreline processes has been initiated at the lowered lake levels, closely related to the pattern of lake level fluctuation. Historically (1965 – 1978) a pattern of rising lake levels in summer-autumn, and falling levels in winter was caused by lake control for electricity generation, reversing the natural seasonal periodicity. Further sealing of the lakebed levels to reduce the amount of discharge during spring flows from Waikaremoana is technically feasible. However, this work is not being considered at this stage.
Genesis Power Limited (as successors to Electricity Corporation of New Zealand Limited) are the current resource consent holders for the hydroelectric power generation from Lake Waikaremoana. This consent was issued by the Hawke’s Bay Regional Council under the Resource Management Act 1991. The resource consent contains a number of conditions for the purposes of avoiding, remedying or mitigating the adverse effects caused by hydro-generation on the lake environment. The Department has been closely involved in Resource Management Act 1991 processes to seek conditions on the resource consent for hydroelectric use of the lake water, that minimise the frequency, extent and duration of lake level variations outside a three metre range and require an avoidance/reversal of seasonal periodicity.
Lake Waikareiti has exceptionally high water quality and is by far the largest of the few lakes in the North Island that remain free of introduced aquatic macrophytes. This provides an important baseline for research on similar lakes. A number of lakelets and tarns occur throughout the park. Those of particular interest include the tarns of Mt Manuoha and Lake Ruapani and the lakelets on Rahui Island and others surrounded by wetland vegetation in the vicinity of Lake Waikareiti. Lake Kiriopukae is of interest with several fluctuating water bodies in an old lakebed covered in wetland vegetation.
The predominantly forested catchments of the park ensures that the water quality of the rivers draining the park is high. The retention of a healthy forest cover also minimises the incidence of flooding in downstream areas and reduces the rate of surface erosion. In this respect, the park performs an important soil and water conservation role benefiting the surrounding areas and communities.
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