LAHAR HISTORY AND HAZARD OF THE TONGARIRO RIVER, NORTHEASTERN TONGARIRO VOLCANIC CENTER, NEW-ZEALAND

Lahar deposits beside the Tongariro River have been mapped and dated u sing andesitic and rhyolitic marker tephras. Coupling the stratigraphi c record obtained with that of the Tongariro and Ruapehu ring plains h as enabled reconstruction of the history of lahars along the Tongariro River. This forms the basis of a lahar hazard map for the entire catc hment. Six lahar hazard zones, with assigned recurrence intervals rang ing from 1 in 35 yr to 1 in >15 000 yr, have been mapped. Lahar deposi ts between the ages of 14.7 and 9.8 ka cover the greatest areas, while younger lahar deposits are confined to lower surfaces closer to the p resent river channel. Holocene lahar deposits along the Tongariro Rive r are not as well preserved as older units, probably because the Holoc ene lahars were confined to a more deeply incised channel and were mor e readily eroded following emplacement. All recorded post-11.85 ka lah ars in the Tongariro catchment were derived from Ruapehu volcano. The Mangatoetoenui Stream has been the conduit for the greatest number of Holocene lahars and for ail of the historic ones. Most of Turangi is b uilt on a surface which has not been inundated by a lahar since c. 10 000 yr ago. However, the area identified where high risk affects the g reatest population is a part of Turangi, which was inundated by a laha r less than 1850 yr ago. The infrastructures at greatest risk include the State Highway 1 bridge across the Mangatoetoenui Stream and the Ra ngipo Dam and power station.