Spillage sedimentation on large river floodplains

Abstract

Active deposition across the floodplains of large rivers arises through a variety of processes; collectively these are here termed 'spillage sedimentation'. Three groups of 11 spillage sedimentation styles are identified and their formative processes described. Form presences on large river floodplains show different combinations of active spillage styles. Only some large floodplains have prominent levees; some have coarse splays; many have accessory channel dispersion and reworking, while still-water sedimentation in lacustrine environments dominates some lower reaches. Infills are also commonly funnelled into prior, and often linear, negative relief forms relating to former migration within the mainstream channel belt.Shuttle Radar Topography Mission (SRTM) and Landsat 8 data are used to map spillage form types and coverage along a 1700 km reach of the Amazon that has an active floodplain width of up to 110 km with a systematic character transformation down-valley. Spillage forms associated directly with mainstream processes rarely account for more than 5% of the floodplain deposits. There is a marked decrease in floodplain point bar complexes (PBC) over 1700 km downstream (from 34% to 5%), and an increase in the prevalence of large water bodies (2% to 37%) and accompanying internal crevasses and deltas (0% to 5%). Spillage sedimentation is likely within the negative relief associated with these forms, depending on mainstream sediment-laden floodwater inputs.Spillage style dominance depends on the balance between sediment loadings, hydrological sequencing, and morphological opportunity. Down-river form sequences are likely to follow gradient change, prior up-river sediment sequestration and the altered nature of spilled loads, but also crucially, local floodplain relief and incident water levels and velocities at spillage times. Considering style distribution quantitatively, as a spatially distributed set of identifiable forms, emphasizes the global variety to spillage phenomena along and between large rivers.

Morphology and dynamics of the intertidal floodplain along the Amazon tidal river

“…In non‐tidal river systems without levees, the elevations of river banks are generally controlled by bar deposition associated with lateral migration of the river channel and subsequent overbank deposition (Wolman and Leopold, ; Lewin et al. ). Where levees are present, their height is limited by the water‐surface elevation during floods (Wolman and Leopold, ; Smith et al.…”

“…The initial elevation of the floodplain behind the levees is likely set by bar deposition associated with lateral migration of the river channel over long timescales, and subsequently modified by overbank deposition (Wolman and Leopold, ; Lewin et al. ).…”

Recent increase of river–floodplain suspended sediment exchange in a reach of the lower Amazon River

“…Along the downstream half of the floodplain margin, the levee is lower, narrow and breached by only a few channels, the two largest of which perennially connect the south‐eastern corner of the floodplain to the main river channel (Figure ). These are typical geomorphological features found on the lower Amazon floodplain, where floodplain channels are restricted by levee building and overbank deposition gradually buries scroll topography (Mertes et al ., ; Dunne and Aalto, ; Lewin et al ., ).…”

“…Rivers transfer sediment to their floodplains during floods through levee breaches and diffuse overbank flows (Dunne et al ., ; Lewin et al ., ), and receive a portion back through floodplain drainage (Alsdorf et al ., ; Trigg et al ., ) that transports sediment remaining in suspension from the main channel along with sediment remobilized from the floodplain by currents and wave action (Schmidt, ). Studies of large river geomorphology, biogeochemistry and ecology share interests in the examination of water and sediment exchange between mainstem rivers and their floodplains (Thorp et al ., ; Lesack and Marsh, ; Jardine et al ., ).…”

Sedimentation of overbank floods in the confined complex channel–floodplain system of the Lower Yellow River, China

“…Floodplains display diverse forms because they evolve from complex processes (Adams, Slingerland, & Smith, ; Constantine, Dunne, Ahmed, Legleiter, & Lazarus, ; Dunne, Mertes, Meade, Richey, & Forsberg, ; Lambert & Walling, ; Latrubesse, ; Lewin & Ashworth, ; Lewin, Ashworth, & Strick, ; Nanson & Croke, ; New & Middelkoop, ; Shen et al, ). During these processes, anthropogenic influences on floodplain development have been of increasing concern, especially in lowland riverine areas (Hudson & Middelkoop, ; Newson, ; Parker, ).…”