CHANNEL-FLOODPLAIN GEOMORPHOLOGY ALONG THE SOLIMOES-AMAZON RIVER, BRAZIL

Citation
Lak. Mertes et al., CHANNEL-FLOODPLAIN GEOMORPHOLOGY ALONG THE SOLIMOES-AMAZON RIVER, BRAZIL, Geological Society of America bulletin, 108(9), 1996, pp. 1089-1107
Citations number
89
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0016-7606
Volume
108
Issue
9
Year of publication
1996
Pages
1089 - 1107
Database
ISI
SICI code
0016-7606(1996)108:9<1089:CGATSR>2.0.ZU;2-5
Abstract
Across the cratonic landscape of Brazil the Solimoes-Amazon River tran sports to its delta plain 1240 Mt of suspended sediment derived from A ndean erosion and reworks another 3200 Mt of floodplain sediments, Dis tribution of these sediments has resulted in a variable along-stream p attern of geomorphology. The upstream reaches are characterized by sed iment erosion in the main channel and deposition in floodplain channel s that are an order of magnitude smaller in discharge than the main ch annel, 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 r estricted by stabilizing, long-term, levee building and floodplain con struction 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 esti mated rates of recycling of floodplain sediments, the modern floodplai n of the Brazilian Amazon could have been recycled in <5000 yr, and is recycled more rapidly in the upstream than the downstream reaches, Th e cratonic interior is interrupted by structural arches that bound int racratonic basins, Four of these arches cross the valley of the main r iver system at intervals of several hundred kilometres and impart a te ctonic imprint on the channel-floodplain geomorphology: at this spatia l scale. Structural arches appear to exert a primary influence by prom oting entrenchment of the river as it passes through zones of deformat ion, thus restricting channel movement, For example, as the river cros ses the Purus arch, the valley narrows to <20 km compared to an averag e of approximate to 45 km, the water-surface gradient decreases, sedim ent is deposited, and yet the rate of channel migration is negligible. Hence, the effect of the arches is to create a landscape where, on th e spatial scale of hundreds of kilometres, the river is confined and e ntrenched in its valley is straight, and is relatively immobile. Local valley tilting apparently unrelated to the arch structures also impri nts the geomorphology. In particular, a tilted valley in the upstream reaches appears to have caused avulsions which have left behind the on ly large-scale, oxbow-type features on the Brazilian Amazon River floo dplain.