BRAIDED-STREAM AGGRADATION ON AN ALLUVIAL-FAN MARGIN - EMERALD LAKE FAN, BRITISH-COLUMBIA

Evolving bar and channel patterns were observed at the distal margin o f an active alluvial fan at Emerald Lake, British Columbia, during pea k midsummer flows. At the fan margin, bed slope is 0.024, sediment is predominantly coarse gravel, flow is shallow and fluctuating, and sedi ment transport and deposition are dominated by chutes and lobes. Flow unconfinement at the exit of rapidly formed shallow scour channels typ ically results in deposition of sediment lobes 0.2 m thick and 10-250 m(2) in area. Closely spaced deposition of a number of these sediment lobes results in aggradation of composite sediment sheets. One such sh eet, monitored daily over a 15 day period, deposited 129 m(3) of grave l over 710 m(2) of adjacent marsh sediment, locally extending the dist al fan margin by 39 m. Thickness of the aggraded bed varied up to 0.37 m, depending on surface topography. During active deposition, individ ual lobe deposits formed simple unit bars that partly projected above the water surface. These bars caused local flow division that, togethe r with avulsion of the dominant channel, initiated a braided stream pa ttern. Complex braid bars composed of several annealed lobe remnants a re 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, discontinu ous, centimetre-thick finer grained layers. High-angle cross-stratific ation was not observed. Since chutes and lobes dominate sediment trans port and deposition in streams at the distal margin of this rapidly ag grading fan, it is likely that similar deposits should be present in m any ancient alluvial fan sequences, but as yet have gone largely unrec ognized.