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Chutes and Lobes: Newly Identified Elements of Braiding in Shallow Gravelly Streams
In-channel features here called chutes and lobes are dominant elements of braiding in the small gravel-bed outwash plain of Hilda Glacier, western Alberta, Canada. Observations of chutes and lobes were made in the field and in a laboratory flume arranged to represent a Froude scale model of the Hilda outwash plain but with steady rather than varying water discharge and sediment input rate. Chute-and-lobe behavior was closely similar between field and laboratory.
Chutes are relatively deep, narrow channels lined with relatively fine sediment, through which coarse clasts move efficiently. A lobe forms at the downstream end of a chute by stalling and jamming of coarse clasts; once initiated, the lobe grows quickly by capturing more coarse clasts until it is several clasts thick and has a downstream slope so steep (up to about 12°) that incision begins, leading to deep dissection and development of one or more new chutes. Incision leaves lobe remnants as emergent gravel-bar topography.
Coarse, poorly sorted sediment together with very shallow flow depths seem to be the essential elements for development of chute-and-lobe behavior.
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