Hamilton's geography is largely the result of successive volcanic ash falls, plus debris, which swept down the Waikato River in at least two massive floods, created by ash blocking the outlet of Lake Taupo. In its present form the landscape originated around 20,000 years ago (20 ka), after the Oruanui eruption at Taupo. The dates given for the eruption vary. A 2007 study said it was between 22.5 and 14 ka. Another in 2004 put it 26.5 ka. After the eruption Lake Taupo rose to about above the present lake. Around 20 ka. the ash dam eroded and the lake rapidly fell some , creating massive floods. The ash they carried formed the main Hinuera Surface into an alluvial fan of volcanic ash, which extends north of Hamilton and drops about from Karapiro. The Waikato changed its course from flowing into the sea at Thames at about that time, possibly just because sediment built up. The peat lakes and bogs also formed about that time; carbon dating gives maximum ages of 22.5 to 17 ka. Due to an ice age, vegetation was slow to restabilise the ash, so dunes formed up to above the local Hinuera surface. The current Waikato valley had cut into the debris by about 12 ka. and was further modified by the 181 AD Hatepe eruption, when again Lake Taupo level fell , generating a flood, equivalent to 5 years' normal flow in just a few weeks. About 800 years ago aggradation began raising the river bed by about .