River, then north through Lake Arapuni and into Lake Karapiro. Pokaiwhenua Stream joins the river in Lake Karapiro. Nine hydroelectric power stations at eight dams extract energy from the river between Taupō and Karapiro. All the lakes in this stretch of the river (apart from Lake Taupō) are artificial. The river leaves the Volcanic Plateau at Karapiro, where it emerges from the Maungatautari Gorge, and flows northwest into the Waikato Basin, flowing through the towns of Cambridge, Hamilton, and Ngāruawāhia. It is joined by its largest tributary, the Waipā River, at Ngāruawāhia. It then flows north through the Taupiri Gorge to enter the lower Waikato region. Further north is Huntly and then Meremere, where the Whangamarino and Maramarua Rivers join it. From Mercer, where the Mangatawhiri River joins it, the Waikato flows west and southwest. Just before its mouth at Port Waikato, the Araroa River joins from the north. Numerous small islands lie in the long, thin delta of the river as it passes through low-lying swampy land between Meremere and the coast, the largest of which is Motutieke Island. There are also over 40 islands between Ngāruawāhia and Tuakau, depending on the level of the river. Maurea Islands, just south of Rangiriri were subject to a restoration trial to test comparative weed treatments, the main weeds being alder and yellow flag iris. In prehistoric times, the Waikato's course has occasionally shifted to flow north through the Hinuera Gap into the Firth of Thames and from there into the Hauraki Gulf / Tīkapa Moana. The most recent occasion this is known to have occurred ended some 20,000 years ago, although it is possible that it also flowed north more recently, until about 1800 years ago. A remnant of this former course can be seen as a spur on Lake Karapiro to the south of the settlement of Piarere. The river's current course is largely the result of the massive Hatepe (Taupō) volcanic eruption of 180 AD. The mean discharge of the Waikato River is , with the highest flows typically occurring in July and August. Specific mean annual floods are low (60–70 L s-1 km-2), and the frequency of events with greater than 3 times the median flow is 0.4 events / year, due to flow regulation and groundwater storage in pumice. Human use As well as being a water and recreation resource, the river was historically a critical communications