The Waihou River is located in the northern North Island of New Zealand. Its former name, Thames River, was bestowed by Captain James Cook.
The river flows north for 150 kilometres (93 mi) from the Mamaku Ranges past the towns of Putaruru, Te Aroha, and Paeroa, before reaching the Firth of Thames at the south end of the Hauraki Gulf near the town of Thames. In its lower reaches, the river and the nearby Piako River form the wide alluvial Hauraki Plains. Just before the river reaches the ocean, State Highway 25 crosses the river over the Kopu Bridge, the longest single lane bridge in the country at 463 metres (1,519 ft) and the only remaining swing bridge on a New Zealand state highway. The bridge was infamous for the queues of vehicles travelling to and from the Coromandel Peninsula until a new two lane bridge was opened in December 2011. Tributaries include the Waimakariri Stream, Waiomou Stream, Oraka Stream and the Ohinemuri River.
In 1879 the Falls of Awotonga were destroyed by 200 pounds (91 kg) of dynamite to free the navigation of the river for shipping. There was a water column of 150 m. Other parts of the river had been cleared in the same manner in previous years.
The crystal-clear waters of the Waihou River provide ideal fishing. The river supports large populations of rainbow and brown trout. A survey conducted in 2009 showed that the upper section of the river supported over 700 fish per kilometre.