River, Coastal and Estuarine Morphodynamics: RCEM 2009 – Vionnet et al. (eds) © 2010Taylor & Francis Group, London, ISBN 978-0-415-55426-8 The Rio Negro and Rio Solimões confluence point – hydrometric observations during the 2006/2007 cycle N. Filizola NMH/CESTU—Universidade do Estado do Amazonas, Manaus, Brazil N. Spínola & W. Arruda UFAM, Manaus, Brazil INPA, Manaus, Brazil F. Seyler & S. Calmant IRD—LMTG, Université de Toulouse, France IRD—LEGOS, Université de Toulouse, France J. Silva COPPE-UFRJ, Rio de Janeiro, Brazil ABSTRACT: The confluence phenomenon between the Rio Negro, with is black waters, and the Rio Solimões, with its suspended white sediments, is well known and attracts thousands of tourists every year near by the city of Manaus, Amazonas—Brazil. From this affluence point on, the Rio takes the legendary name of the Amazon River. In spite of the wide interest for this phenomenon, there are few studies concerning its hydrological functioning. A backwater effect takes place at the confluence, as the Solimões waters play the role of a hydraulic dam slowing down the Negro waters (Meade et al., 1991). The objective of this study is to narrow the knowledge gap by describing better this effect by hydro-acoustic measurement results on the 4 periods of hydrological cycle (2006/2007). The results in this text are supported by new discharge data now accessible. The data shows that the Rio Negro, whose levels are controlled by Rio Solimões water levels, has lower discharge in the period of water rise, than during the period of low water levels. This demonstrates a different dynamic between water discharge and water levels in that region. The data collected also helps distinguishing some of the main features from those two big rivers at different periods of the hydrological cycle: rising waters, flood peak, decreasing waters and low waters. The differences between these two rivers are emphasized mainly on parameters such as: flow velocities, water discharge and water slope. These differences still persists even considering the strong seasonality that controls the Amazon rivers hydrology. 1 INTRODUCTION The Amazon River’s contribution to the world fresh water volume is estimated about 16% (Milliman and Meade, 1983). The Rio Amazonas basin area covers 6 million km 2 and its long period average total flow is estimated at 209.000 m 3 ·s −1 (Table 1). The two biggest contributor basins for the Amazon River (at Manaus) are: the Solimões basin (near Manacapuru), which rep- resents approx. 36% of the total area of the Amazon basin, and the Rio Negro basin (near Manaus) which represents 11% (Molinier et al., 1995). In terms of their contribution to the total mean discharge at the mouth of the Rio Amazonas the Rio Solimões is responsible for 49% of fresh water and the Rio Negro contributes 14% (Molinier et al., 1995). The hydrological regime of the Rio Negro is consid- ered an Equatorial type according to the classification of Jean Rodier (1964), adapted by Mollinier et al. Table 1. Hydrological reference data from the Rio Ama- zonas basin, the Rio Negro and the Rio Solimões at some key gauge stations. Óbidos is the last gauged station before the Amazon mouth. Jatuarana is a station downstream the confluence. Source: Filizola, 1999 e Molinier et al., 1995. River basin A* (km 2 ) Q* (m 3 · s −1 ) Solimões at Manacapuru 2.147.740 103.000 Negro at Manaus 696.810 28.400 Amazonas at Jatuarana 2.854.300 131.600 Amazonas at Óbidos 4.618.750 168.700 Amazonas (Total) 6.112.000 209.000 (A*) Drainage area. (Q*) Mean annual discharge. (1995) for the Amazon region. This type of regime is locally characterized by the occurrence of two max- imum discharge peaks during the year. The first peak occurs in the first 90 days of the year. It is weak and 1003