Floodplain morphology, sedimentology, and development processes of a partially alluvial channel

• A facies model is presented for a coarse-grained, partially alluvial channel.
• Overbank deposition of fine sediment is emphasized over point bar accretion.
• Channel abandonment occurs primarily through upstream incision of flood channels.
• The model is fitted within a new 3-dimensional floodplain classification framework.
• Confinement is important to floodplain development in partially alluvial channels.

The floodplain morphology, sediment deposits, and development mechanisms of a partially alluvial, low-moderate energy channel flowing over a mixed gravel/cobble-till bed are investigated and compared to existing ideas of floodplain development. The findings partially support the idea of a floodplain developed through lateral accretion capped with vertically accreted sediments as predicted by the energy-based classification scheme of Nanson and Croke (1992), though oblique accretion and partial channel avulsion are also important. Channel migration consists of limited cross-valley migration and downstream meander translation. Because of low channel sinuosity, well-formed neck cutoffs are rare, and instead the channel cuts headward along the insides of confined or underdeveloped meander bends forming a localized anabranching pattern. The floodplain architecture can be divided into gravel bar and bed deposits (GB), lateral accretion deposits (LA), overbank deposits (FF), and abandoned channel deposits (FF(CH)), which are described with four alluvial facies. Owing to the limited supply of coarse and fine sediment, none of the architectural elements are particularly thick, with total floodplain thickness being < 3 m. Floodplain development for partially alluvial channels is compared within a new floodplain discrimination framework. Comparisons with common facies models of single-thread, coarse-grained channels show important differences that suggest that the floodplain deposits and formative processes described herein represent a subset of single-thread systems that may be common in partially alluvial channels, particularly in slightly sinuous, coarse-grained channels of low-moderate energy with partly confined floodplains.