Dynamic relations for the deposition of sediment on floodplains and point bars of a freely-meandering river

Fluvial features such as floodplains and point bars are built by sediment deposition and sculpted by erosion. Long-term measurements (38 yr) of the cross-section topography of active floodplains and point bars along the freely-meandering Powder River in southeastern Montana, USA (mean daily discharge of 12.5 m³ s⁻¹), were used to develop dynamic relations between annual sediment deposition and peak-flood discharge. Five floodplain sections and five point-bar sections were selected from 24 cross sections along a 90-km study reach. At each cross section the sediment deposition volume per unit streamwise distance was computed as the difference between two topographic surveys made in consecutive years.

Snowmelt floods were found to be the dominant annual process. However, other processes such as flash floods, ice breakup floods, and autumnal floods were important episodically. The dynamic relations were linear for both fluvial features. The snowmelt deposition-discharge relations showed, in general, that point bars were about two times more efficient at trapping sediment than floodplains. Each relation had a discharge threshold that must be exceeded before sediment was deposited. Although these discharge thresholds for floodplains and point bars had essentially the same mean value (69 and 71 m³ s⁻¹, respectively), they represented different processes. Thresholds for other rivers will probably differ from those for Powder River because of different channel geometry and sediment characteristics.