Abstract

Evolving bar and channel patterns were observed at the distal margin of an active alluvial fan at Emerald Lake, British Columbia, during peak midsummer flows. At the fan margin, bed slope is 0.024, sediment is predominantly coarse gravel, flow is shallow and fluctuating, and sediment transport and deposition are dominated by chutes and lobes. Flow unconfinement at the exit of rapidly formed shallow scour channels typically results in deposition of sediment lobes 0.2 m thick and 10-250 m2 in area. Closely spaced deposition of a number of these sediment lobes results in aggradation of composite sediment sheets. One such sheet, monitored daily over a 15 day period, deposited 129 m3 of gravel over 710 m2 of adjacent marsh sediment, locally extending the distal fan margin by 39 m. Thickness of the aggraded bed varied up to 0.37 m, depending on surface topography. During active deposition, individual lobe deposits formed simple unit bars that partly projected above the water surface. These bars caused local flow division that, together with avulsion of the dominant channel, initiated a braided stream pattern. Complex braid bars composed of several annealed lobe remnants are gradually exposed as waning discharge becomes confined to adjacent chutes. The newly aggraded fan margin is mainly composed of massive to crudely stratified imbricated gravel with interstratified, discontinuous, centimetre-thick finer grained layers. High-angle cross-stratification was not observed. Since chutes and lobes dominate sediment transport and deposition in streams at the distal margin of this rapidly aggrading fan, it is likely that similar deposits should be present in many ancient alluvial fan sequences, but as yet have gone largely unrecognized.

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