Transformation, internal stratification, and depositional processes withina channelised, multi-peaked lahar flow

On 25 September 1995, phreatomagmatic explosions through Crater Lake at Rua pehu volcano, New Zealand, generated a closely spaced sequence of lahars. F rom direct observations of the flows and timely description of their deposi ts, we infer these debris flows transformed to hyperconcentrated streamflow s not by dilution with incorporated water they overran, as previously propo sed. Rather, the described debris flows diluted by selective deposition of their coarse clasts as they thinned and decelerated while spreading >700 m laterally over the Whangaehu fan. Deposits recording this transformation ar e veneering (<100 mm thick) layers of muddy sandy gravel interspersed with many boulders and cobbles. Downstream of their transformation to hyperconce ntrated streamflows, ephemeral near-channel deposits indicate the flows wer e vertically stratified. A new depositional model for these hyperconcentrat ed streamflows includes a basal, coarse, sediment-concentrated "channel flo w" that emplaced transitory near-channel sediment wedges. The near-channel sediment was bouldery, massive, and poorly sorted, like debris-flow deposit s elsewhere in the Whangaehu catchment. The upper and marginal parts of the lahars (the surface layer) were diluted, finer hyperconcentrated flows tha t left voluminous overbank deposits. The overbank sediment is poorly sorted gravelly sand, with some degree of horizontal bedding, like other hypercon centrated flow deposits elsewhere in the catchment. The rapid erosion of ch annel-flow deposits within days to months of the events indicates that geol ogic records will only preserve lateral-flow deposits of such lahars. Hence , long after an event, interpretation of hyperconcentrated streamflow mecha nisms from geologic deposits can be misleading without the near-channel rec ord.