En masse debris transport in a mountain stream

Accumulations of stored sediments in a sub‐alpine environment of a front range of New Zealand's Southern Alps closely resemble sieve deposits previously described from southeastern California. The locations of such deposits, within active river channels, has important implications for long term sediment yields from catchments where such deposits occur. The catastrophic failure of a sieve deposit in the Torlesse Stream catchment, the site of an earlier investigation of mountain stream sediments, resulted in a sediment yield equivalent to a third of the total yield recorded over an eight‐year period. Derived bed‐load transport rates represent, at a minimum, a four‐fold increase over calculated average transport rates. Such deposits are only likely to fail during extreme low frequency events since their highly permeable gravels allow for the continuation of stream flow underneath the bulk of the deposit and restricts significant increases in moisture content within the deposit. The loss of water is responsible for the inhibition of flowage‐type mass movement transport mechanisms wherein available water contributes to the maintenance of positive pore pressures. For the example studied here the percentage of silt and clay is less than 3 per cent of the total sample while sorting values (σ 1 ) are generally less than 2.0ϕ. These values are approximately one‐third and one‐half respectively, of typical values obtained for these parameters from debris flow deposits. The feature described here is believed to have originated through failure of a higher altitude perched scree‐field. This mode of origin would account for the distinctive sediment size parameters.