Occurrence of landslides in the Valley of the Solimões-Amazon River and its tributaries, causes of formation and environmental implications
by
Elena Franzinelli
Department of Geosciences, Federal University of Amazonas, Brazil
Coauthors: Hailton Igreja (Department of Geosciences, Federal University of Amazonas, Brazil)

Numerous and diversified are the landslides that occur in the valley of the Solimões-Amazon River and its tributaries. Their extent varies from a few to some hundreds of meters. The material of these landslides consists of friable fine sands of the floodplain of the same rivers or of outcrops of the Alter do Chão or Solimões Formation that in the medium and lower Amazon Basin respectively, form the substratum of the floodplain.

The most important parameters that contribute to the formation of these landslides are the textural characteristics of the sediments, the topography of the levees and their vegetal covering, the tropical climate and the water. The principal causes of the landslides in the Amazon region are the erosive actions of surface and ground water, the power of the current of the river water, and very importantly the influence of neotectonics.

The locally called “Terras Caídas” (Earth fall) are small landslides of friable deposits in unstable equilibrium in the inclined surfaces of the levees. They are very frequent along the river banks in Amazonia. The mechanisms that produce them are linked to the hydrodynamic forces of the infiltrating water and the action of the erosive running water of the rivers against the levee.

All the channels of the rivers of Amazonia are related to neotectonic features (Igreja, l998), thus the influence of neotectonics is a decisive factor in the larger landslides in Amazonia.We mention three events of large landslides caused by neotectonisms: The first occurred 30 years ago in the area of the confluence of the Içá and Solimões Rivers and destroyed one district of the town of Santo Antônio de Içá, the second took place in 1970, near the confluence of the Japurá and Solimões Rivers. It caused the fall of a lot of material with consequent changes in the geomorphology of the channels and lakes nearby. The third, the most impressive and devastating, occurred three years ago, on a north side of the Negro River, in a port to the east of the city of Manaus, in a zone where evidences of neotectonisms exist (Franzinelli e Igreja, 1990) and caused damage to the port and boats and a loss of lifes.

Franzinelli, E.; Igreja, H. Utilização de sensoriamento remoto na investigação da área do Baixo Rio Negro e Grande Manaus. VI Simp. Brás. Sens. Rem. Anais, V. 3, p. 641-648. Manaus, Amazonas, 1990.

Igreja, H. Aspectos do modelo neotectônico da placa sul-americana na província estrutural amazônica, Brasil. Tese, UFAM, Manaus, 1998.

Date received: November 18, 2004