Crevasse splay (crevasse channel fill)

Position: Floodplain

 

Form

A sediment tongue fed by a crevasse channel that breaches the levee. Crevasse splays have a lobate or fan-shaped planform with distal thinning away from the levee. The surface of the crevasse splay may have multiple distributary channels. May produce hummocky topography. Composed of bedload material, predominantly sand, sometimes gravel. The crevasse channel fill has a symmetrical, lenticular geometry and low width-depth ratio. Upward-coarsening gradation of grain sizes is common, as is proximal-distal gradation away from the channel.

 

Process description

Crevasse channels breach and erode the levee taking bedload materials from the primary channel and conveying them onto the floodplain at high flood stage. Deposition reflects the rapid loss of competence beyond the channel zone. Flow velocity is sufficient to carry relatively coarse material, which is spread outward onto a fan-shaped area of floodplain which thins away from the levee. The angle of trajectory increases with high levee backslopes and/or decreases with higher flow velocity. Crevasse channel fills represent bedload plugging of old crevasse channels, indicating an aggradational environment. Their formation may be linked with the formation of an alluvial ridge.