A distinct white bed 1–2 cm thick is present within backwater deposits at the upstream end of the Turbid Creek debris fan, southwestern British Columbia, Canada. This white bed and the enclosing backwater deposits accumulated within an impoundment of Squamish River caused by a large ca. 4800 BP debris avalanche originating from Mount Cayley, a Pliocene–Pleistocene volcano. The white bed is composed of silt and clay detritus and resembles tephra. About 5% of the grains exhibit optical characteristics consistent with volcanic glass. The remainder, however, are mineral and lithic particles (mainly cristobalite and Na-feldspar, with minor quartz, and trace amphibolite, mica group, and chlorite). The white bed is interpreted to be dust derived from the debris avalanche and washed into the impoundment, based upon (i) the high proportion of mineral and lithic grains within the bed, (ii) the contemporaneity of the bed and the debris avalanche, (iii) the lack of Holocene volcanic activity at Mount Cayley, and (iv) the significant age difference between the bed and known tephras in southwestern British Columbia.