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Abstract

Sedimentological studies on the Ilha de Marchantaria an island in the Amazon river near Manaus reveal the existence of four different structural main units.
A: Sandy channel bars consisting of giant ripples constitute deposits for the formation of islands or newly formed areas of the Varzea.
B: Gradual accumulation on the channel bars leads to the formation of point bar ridges which consist chiefly of small-scale ripples.
C: Between point bar ridges of different ages there exist swales. During rising water level the river water flows from the downward side into the swales where finegrained sediments are deposited.
D: Permanent lakes are formed mainly in the centre of the islands. During low water periods
the lakes may be cut of from the river.
During high water periods when the whole island is flooded by the river, additional sedimentation takes place especially in the upstream area of the island. There is however, extensive erosion of the banks on the upstream side of the island. By erosion of the upstream end and sedimentation processes
on the downstream end the island moves slowly downstream.