Palaeochannel (prior channel, abandoned or ancestral channel)
An old, inactive channel on the floodplain. May be partially or entirely filled. Extends over more than one meander wavelength (thereby differentiating it from a meander cutoff). Can have a wide range of planforms, from elongate and relatively straight to irregular or sinuous, reflecting the morphology of a former primary channel. Low-sinuosity palaeochannels may be overprinted with flood channels. May have an upward-fining fill comprising a channel lag of coarser material with finer, suspended-load materials atop.
Caused by a sudden shift in main channel position (avulsion), generally to a zone of lower elevation, abandoning a channel on the floodplain. The palaeochannel may subsequently fill with suspended-load sediments derived from overbank flooding. They record palaeoplanform and geometry of the avulsed channel. If this is markedly different from the contemporary channel, it may indicate a shift in sediment-load, discharge or distribution of flood power within the system.