Waikato means “flowing water” and the Waikato River, 220 miles long, is the longest river in New Zealand, rising in the snows and ice fields of Tongariro National Park. Before entering Lake Taupo it is known as the Tongariro River for part of its length. On leaving Lake Taupo it flows in a well-defined bed through steep uplands before reaching the flat plains of the middle Waikato basin or Hamilton lowlands. After passing through the Taupiri Gorge it reaches the flat country of the lower Waikato and finally enters the sea at the now defunct port of Port Waikato. Because of the effect of Lake Taupo, and the many artificial lakes along its course, river rises are not spectacular; however, the country adjacent to the river in the lower Waikato is low, and considerable inundation can occur. Serious floods usually result from a fresh from its main tributary, the Waipa River, coming at a time of high base flow in the Waikato. The greatest known flood occurred in 1907 when the peak discharge at the Taupo outfall was 7,350 cusecs, and that at Mercer in the lower Waikato was estimated as being 60,000 cusecs. Minimum flows of less than 7,000 cusecs have been recorded in the latter area.

At one time the river was an important access route for the Maoris and early European settlers, and during the Waikato Wars several Maori pas along the river fell victim to small armoured gunboats. Owing to a shifting sandbar at the mouth, Port Waikato near the coast has not been in use for some time, but a small amount of internal trade is still done in barging sand and shingle as far up stream as Hamilton.