Impact of the Amazon tributaries on major floods at Óbidos.

Major flooding of the Amazon river (with discharge over 250 000 m 3 s -1 ) is analysed, using daily discharge at (a) Obidos, the lowest gauged station on the Amazon River, 800 km from the ocean, (b) six gauge stations located on the main tributaries and characterized by different discharge regimes. During the 1984-2001 period major flooding on the main stream was related to the following features: (a) greater than usual high water in the Solimoes River, the largest tributary, (b) delayed discharge peaks in the southwestern tributaries and the Amazonas River (Peru) and/or advanced discharge peaks on the Negro-Branco River, (c) unusual February-April discharge peaks in the western and northwestern tributaries, particularly in the Negro River. These two last features contribute to the simultaneous inflow of a great quantity of water from all the tributaries in April-May and may favour major flooding of the Amazon River at Obidos.