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Channel-floodplain geomorphology along the Solimoes-Amazon River, Brazil

Leal A. K. Mertes, Thomas Dunne and Luiz A. Martinelli
Channel-floodplain geomorphology along the Solimoes-Amazon River, Brazil
Geological Society of America Bulletin (September 1996) 108 (9): 1089-1107

Abstract

Across the cratonic landscape of Brazil the Solimoes-Amazon River transports to its delta plain 1240 Mt of suspended sediment derived from Andean erosion and reworks another 3200 Mt of floodplain sediments. Distribution of these sediments has resulted in a variable along-stream pattern of geomorphology. The upstream reaches are characterized by sediment erosion in the main channel and deposition in floodplain channels that are an order of magnitude smaller in discharge than the main channel. Sediment deposition in and migration of the floodplain channels erases oxbow lakes of the main channel and yields an intricate scroll-bar topography that forms the boundaries of hundreds of long, narrow lakes. In contrast, downstream reaches are characterized by channels restricted by stabilizing, long-term, levee building and floodplain construction dominated by overbank deposition. Overbank deposition buries the scroll-bar topography, resulting in a flat floodplain covered by a patchwork of large, more equant, shallow lakes. On the basis of estimated rates of recycling of floodplain sediments, the modern floodplain of the Brazilian Amazon could have been recycled in <5000 yr, and is recycled more rapidly in the upstream than the downstream reaches. The cratonic interior is interrupted by structural arches that bound intracratonic basins. Four of these arches cross the valley of the main river system at intervals of several hundred kilometres and impart a tectonic imprint on the channel-floodplain geomorphology at this spatial scale. Structural arches appear to exert a primary influence by promoting entrenchment of the river as it passes through zones of deformation, thus restricting channel movement. For example, as the river crosses the Purus arch, the valley narrows to <20 km compared to an average of nearly equal 45 km, the water-surface gradient decreases, sediment is deposited, and yet the rate of channel migration is negligible. Hence, the effect of the arches is to create a landscape where, on the spatial scale of hundreds of kilometres, the river is confined and entrenched in its valley, is straight, and is relatively immobile. Local valley tilting apparently unrelated to the arch structures also imprints the geomorphology. In particular, a tilted valley in the upstream reaches appears to have caused avulsions which have left behind the only large-scale, oxbow-type features on the Brazilian Amazon River floodplain.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 108
Serial Issue: 9
Title: Channel-floodplain geomorphology along the Solimoes-Amazon River, Brazil
Affiliation: University of California at Santa Barbara, Department of Geography and Institute for Computational Earth System Science, Santa Barbara, CA, United States
Pages: 1089-1107
Published: 199609
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 95
Accession Number: 1996-074689
Categories: Geomorphology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: CAMREX Contrib. No. 79
Illustration Description: illus. incl. 1 table, sketch maps
S07°30'00" - N00°00'00", W75°00'00" - W50°00'00"
Secondary Affiliation: Centro de Energia Nuclear na Agricultura, BRA, Brazil
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 199623
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