Hinuera course. After the huge Oruanui eruption 27,000 years ago ignimbrite was showered all over the North Island to a thickness of . A new lake was formed – Lake Taupō. The water accumulated until a new outlet was forced above the present lake level, near Waihora Bay. Over the next few thousand years the bed of the river was raised by large amounts of eruption debris. Then the original, blocked outlet suddenly gave way; the lake level fell as about of water and debris poured out in a catastrophic breakthrough flood causing the river to change course near Piarere. The Hinuera Gap and Waitoa River are evidence of the river's former course. The water level dropped quickly and the river stayed in this new course through the Maungatautari gorge and Hamilton Basin. Deposits show that the Waikato River was already in the Waikato Basin 21,800 years ago. Course The river starts in the form of many small streams on the eastern slopes of Mount Ruapehu. The Mangatoetoenui Glacier (once also called the Waikato Glacier) is one of the principal sources. The southernmost tributary is called the Upper Waikato Stream. The Waipakihi River joins the Waikato from the Kaimanawa Mountains to the west. From the point where the river meets the Waihohonu Stream, down to Lake Taupō, it has been formally named the Tongariro River since 1945. The Poutu Stream joins from Lake Rotoaira to the east, as a tributary of the Tongariro, which flows northward, with State Highway 1 in parallel, through the town of Tūrangi, and into the southern side of Lake Taupō. Extensive engineering of lakes, tunnels and canals are used to generate hydroelectric power in the Tongariro Power Scheme. The Waikato River flows out of Lake Taupō at the town of Taupō in Tapuaeharuru Bay at the northeast end of the lake. It flows northeast past the town, alongside State Highway 1, to the Huka Falls. State Highway 5 runs more or less parallel to the river as it flows further northeast. About from the lake, the river flows west and into the southern end of Lake Ohakuri. It exits from the northwest end of that lake and flows west through the small Lake Atiamuri and into the long east–west oriented Lake Whakamaru, with State Highway 30 following its course. It passes northwest through Lake Maraetai and Lake Waipapa, where it is joined by the Waipapa