The Piako River is a lowland river system that drains into the Firth of Thames on the North Island of New Zealand. Together with the Waihou River, it is one of the two main rivers systems which drains the Hauraki Plains. It is the dominant river system in the Matamata-Piako District, and on its journey it passes through the towns of Morrinsville and Ngatea.

The Waihou River is located in the northern North Island of New Zealand. Its former name, Thames River, was bestowed by Captain James Cook in November 1769, when he explored 14 mi (23 km) of the river from the mouth. An older Māori name was "Wai Kahou Rounga". A 1947 Geographic Board enquiry ruled that the official name would be Waihou.