Large lake floods scoured New Zealand.

Portions of New Zealand's North Island, like many volcanic regions, have experienced immense floods when lakes filling the crater's of dormant volcanoes burst through craters' rims. Now, scientists analyzing signs of erosion in the area have estimated the size of some of those powerful deluges.

Some of the largest such floods originated in Lake Taupo, says Vern Manville, a geologist with the Institute of Geological and Nuclear Sciences in Taupo, New Zealand. That 616-square-kilometer lake occupies the hole left when a volcano erupted about 1,800 years ago. In the decade or so just after that eruption, the surface of the lake rose to an altitude about 34 meters higher than today's level, says Manville. When the water eventually breached a large dam of ash along the crater rim, about 20 [km.sup.3] water rushed out and down the crater's slopes. In just a few weeks, the torrent--estimated to have carried up to 30,000 [m.sup.3] of water per second--chewed a 12-km-long spillway and deposited layers of wet ash up to 17m thick on the surrounding floodplain.

A similar but even larger flood occurred after an eruption 26,500 years ago,ff when about 60 [km.sup.3] of water spilled from the lake, says Manville. That deluge ripped large boulders out of solid rock about 80 km downstream and carried them several kilometers further, suggesting a peak flow rate above 100,000 [m.sup.3]/s.

Today, the crater of nearby Mount Ruapehu, which last erupted in 1996, holds a rising lake. At current rates of water accumulation, the lake could breach the crater rim by 2007 and release a flood of up 1.5 million [m.sup.3] onto populated areas. But such a catastrophe need not occur if engineers stop the rise by constructing an erosion-resistant spillway in the natural dam, notes Manville. In 2002, engineers in the Philippines did just that at the lake accumulating inside the carter of Mount Pinatubo, which erupted in 1991.--S.P.