Three lahars were sampled in the Whan-gaehu River on the eastern flank of Ruapehu volcano, New Zealand, at 23.5 and 42 km from the source as they were flowing on September 27 and 29 and October 6, 1995. The lahars were generated by water explosively ejected from the highly mineralized Crater Lake. We used the chemical contrast between the lahars and resident stream water in their paths to describe a four-phase model of a noncohesive lahar wave in a river channel: (1) ambient stream water pushed ahead of the lahar in a process of miscible displacement due to hydrodynamic dispersion; (2) a zone of mixing between the stream water and the lahar that increases in length with distance from source; (3) a remnant of the original lahar, least diluted by stream water, that decreases in length and dilutes downstream; (4) the tail of the lahar surge. Peak discharge occurs at the end of phase 1 as water is pushed ahead of the lahar. Peak sediment concentration occurs at the end of phase 2, where debris entrainment by the flow is at its greatest (i.e., in front of the lahar proper). Deposits record only phases 2 and 3 of the lahar wave, phase 1 flow left only a tide line of organic debris, and phase 4 deposits were rapidly eroded by later streamflow. Downstream dilution by stream water eventually caused transformation of phases 2 and 3 of the lahars from debris flow to hyperconcentrated streamflow and then to normal streamflow as the flows became progressively finer grained and more turbulent.

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