Report: TR2005/49

Author: Graeme Smart (NIWA)

Abstract

This report is in response to public concern over the perceived high level of the Lower Tongariro. It describes the general state of the Tongariro River from Rangipo to the mouth, noting changes over the last 40 years and suggests what will happen in the future. It lists possible counter measures to manage the river from Turangi to the mouth. A bibliography of known Tongariro River studies is also included.

The Tongariro is a volatile river which undergoes significant channel changes in response to floods and eruptions. The river transports vast amounts of sediment through its upper reaches and deposits the sediment on its delta from Turangi downstream.

Visual inspections of the lower delta show the river is close to breaking out of its present channel to find a new course to Lake Taupo. There is a significant amount of water being lost from the ‘normal’ river channel, even during moderate freshes. Floodwaters spill from the river upstream of De Latours Pool eastwards to Stump Bay and west towards the Tokaanu Tailrace via Deep Stream. The most likely future breakout route is from Downs Pool to Tokaanu Bay via Deep Stream.

The dominant factors influencing the growth of the lower delta have been the frequency and magnitude of floods and eruptions, the level of Lake Taupo during floods and willow tree growth on the delta. The effects these factors have had on the delta building process can be considered either beneficial or detrimental depending on whether one is considering wildlife habitat or human infrastructure on the delta.

The Higher Lower Tongariro [PDF, 1.3 MB]