The Waikato River is 442 km in length – New Zealand’s longest river.
Earliest historical records suggest that, for at least a million years, a large lake in the centre of the North Island, Lake Huka, was drained by the ancestral Waikato River. This river flowed through the Hinuera Valley and eventually entered the Hauraki Gulf.
A more recent history
A more recent history began after a huge eruption 26 500 years ago that destroyed Lake Huka and formed a large caldera – the present day Lake Taupō. Initially, the lake overflowed into the Mangakino Stream (a tributary of the Waikato River) and became the outlet for thousands of years. About 22 500 years ago, the lake broke out of its basin near the present outlet, carving a course through the Reporoa Basin. As the lake level fell, the Mangakino Stream outlet dried up.
Over time, the river flowed into the Hamilton Basin, depositing alluvium in the valleys. Many tributary streams became dammed by the flood of alluvium, giving rise to the formation of many small lakes (some of which exist today). Eventually, the debris fell away, and the river changed from a braided fan-building format to a single shallow course flowing down the fan. In time, this course became choked by flood debris, and a new course was formed. This was repeated a number of times. 17 000 years ago, debris had further decreased, and the river deepened its bed and became trapped in its present course.
The Taupō eruption of 230 CE again dammed the outlet of Lake Taupō. The breakout flood that followed filled the whole length of the river valley with pumiceous debris. Subsequent degrading deepened the river, leaving low terraces on either side covered with pumice
The river today
The upper river
Today, the river is fed by over 17 000 km of tributary streams and drains a catchment area of 11 013 km2. The river initially travels through a narrow valley and has a steep gradient, swift flows and many rapids. Eight hydroelectric dams occur on the steep-gradient section between Taupō and Karāpiro (often referred to as the upper river). Impoundments in the hydro lakes have increased the time of water in the river from around 5–6 days (before the dams) to about 40 days under low water flow conditions and 16 days under high water flow conditions.
From Karāpiro to Ngāruawāhia
At Karāpiro, the river emerges into the Hamilton Basin. From here, it makes its way to Port Waikato – 150 km downstream of Karāpiro. The river enters the basin above Hamilton (at Piarere) and follows a set course that becomes progressively shallower towards Taupiri. It flows through Hamilton – New Zealand’s fourth largest city. The Waipā River, the largest tributary of the Waikato, enters the Waikato at Ngāruawāhia. The Waipā can contribute up to 50% of the Waikato flow. Sediment from the Waipā River reduces the clarity of the Waikato north of Ngāruawāhia.
From the Taupiri Gap to the sea
The Waikato River leaves the Hamilton Basin at the Taupiri Gap (between two hill ranges), near Huntly. North of Huntly, the river flows across what is essentially a flood plain interspersed with shallow lakes and wetlands (such as the Whangamarino wetland and Opuatia wetland). Flood control works, land clearance, sand mining and wetland drainage have altered the nature of this flood plain, once characterised by a wide shallow channel where the flow interacted with backwater channels, flood plain lakes and peat wetlands. The environment provides important habitat for a wide range of plants and animals – some introduced and some native, including some rare and endangered
West of Tuakau, the river passes through a delta made up of a network of channels – formed among islands of deposited sediment. The river then enters Maioro Bay and the Tasman Sea through a narrow channel at Port Waikato. This lower section is influenced by daily tides – causing changes in water levels, sometimes as far up as Rangiriri, 63 km away. The lower section is a wetland complex supporting a wide range of bird and fish species, including habitat for whitebait spawning. The salinity of the water is affected for about 6 km upstream.