Originally the Guapiaçu and Macacu rivers had separate outlets into the bay. However, in the 1950s the no longer existing Departamento Nacional de Obras e Saneamento (DNOS, National Department of Works and Sanitation) rectified and channelized several rivers in the region to avoid flooding, reduce the habitat for mosquitoes and, therefore, water-related diseases such as malaria and yellow fever (Cáceres Benavides et al., 2009). These channelization works aimed at gaining fertile land for crop production as well. During the implementation of these infrastructure measurements the natural course of the Macacu river was diverted and artificially connected, through the Imunana channel, to the Guapiaçu river to form a new river basin (UFF/FEC, 2010). For instance, these works changed the hydrological processes taking place within the basin increasing channel flow velocity and river bank erosion.
The largest rivers, Macacu and Guapiaçu, have their headwaters in the northern mountain range and drain the basin on a NE-SW orientation meeting before the Imunana channel. After the encounter, the two rivers flow into the sea before receiving water from another important tributary (Guapimirim river).

The morphology of the main rivers can be divided into three main segments (Cáceres Benavides et al., 2009). The first one where the headwaters found their origin at the Serra do Mar with high slopes, rocks as riverbed material, many waterfalls and Atlantic forest as main vegetation. The second segment, a transitional one, occurs between the mountain range and the lowlands, with less slope, sandier riverbed and lowland trees (adapted to more humid soils) as main vegetation. The third and last segment goes through the lowlands (often flooded in the rainy season) into the sea. Meanders cannot be found due to the channelization works done in the 1950s. Riverbed is sandy and main vegetation is pasture, agriculture and small trees (Cáceres Benavides et al., 2009).
As mentioned above, annual rainfall concentrates in summer while winters are rather dry periods. Being precipitation the main driver of the hydrological cycle, streamflow within the basin is consistent with rainfall patterns. According to the available streamflow data (at Parque Ribeira, Macacu river). January is the wettest month with an average streamflow of 14.0 m³•s-1 while August is the driest month with an average of 4.2 m³•s-1. The annual average at this point (middle stream) is 9.1 m³•s-1.

Groundwater is another important hydric resource. According to Ecologus-Agrar (2003) two main aquifer types can be found in the region: sedimentary and crystalline. Within these categories several formations can further be identified. The table shows the approximate area, thickness and lithology of the four identified aquifers within the GMRB. Groundwater is not a widely used resource in the region for drinking water or irrigation. According to UFF/FEC (2010) there are 15 registered wells in the GMRB with an average extraction rate of 12 m³•h-1.

Table: Main aquifers in the GMRB (UFF/FEC, 2010).

Aquifer Area [km2] Lithology Type Thickness [m]
Formação Macacú 62 Intercalated sediments: clay, silt, sand and conglomerated Porous 200
Alluvial 218 Sand, slit, clay and organic matter Porous 100
Fluvial-lake marine sediments 122 Sand-clay levels with clay and silt layers Porous 100
Crystalline rocks 863 Gneiss, Migmatite, Granite and alkaline rocks Fractured  --


References and further reading:

Almeida, F. F. de M., Carneiro, C. dal R. (1998) Origem e Evoluçáo da Serra do Mar, Revista Brasileira de Geociências, 28(2), 135–150,

Barreiro, M., Chang, P., Saravanan, R. (2002) Variability of the South Atlantic Convergence Zone Simulated by an Atmospheric General Circulation Model, Journal of Climate, 15(7), 745–763,<0745:VOTSAC>2.0.CO;2, accessed: 20.12.2013.

Benavides, Z. et al. (2009) Consumo e Abasticemento de Água nas Bacias Hidrográficas dos Rios Guapi-Macacu e Caceribu - RJ.

Ecologus-Agrar (2003) Plano Diretor de Recursos Hídricos da Região Hidrográfica da Baía de Guanabara, p. 3087.

Fernandes, N. F. et al. (2010) Rio de Janeiro: A Metropolis Between Granite-Gneiss Massifs, P. Migón, ed. Geomorphological Landscapes of the Worl, Springer, p. 387,, accessed: 21.12.2013.

Ferrari, A. L. (2001) Evolução Tectônica do Graben da Guanabara, Biblioteca Digital de Teses e Dissertações da USP,, accessed: 25.09.2013.

Fidalgo, E. C. et al. (2008) Uso e Cobertura da Terra na Bacia Hidrográfica do Rio Guapi-Macacu, Rio de Janeiro - Brazil.

Finotti, R. et al. (2012) Variations in structure, floristic composition and successional characteristics of forest fragments of the Guapiaçu river basin (Guapimirim/Cachoeiras de Macacu, RJ, Brazil). Acta Botanica Brasilica, 26(2), 464–475,, accessed: 23.12.2013.

IUSS Working Group WRB (2006) World reference base for soil resources 2006, Rome.

Lumbreras, J. F. (2008) Relacoes solo-Paisagem no noroeste do estado do Rio de Janeiro: Subsídios ao Planejamento de uso sustentável em áreas de relevo acidentado do bioma Mata Atlântica, Universidade Federal de Rio de Janeiro.

Naegeli, F. E. (2010) Evaluation of Forest Fragmentation and Land Use Change Patterns using Remote Sensing Techniques and Field Methods.

Penedo, S. et al. (2011) Implementation of a Hydro-climatic Monitoring Network in the Guapi-Macacu River Basin in Rio de Janeiro Brazil, XIVth IWRA World Water Congress, Porto de Galinhas, Brazil, p. 12, of a Hydro-climatic Monitoring Network in the Guapi-Macacu River Basin in Rio de Janeiro Brazil.pdf.

Pinheiro, H. S. K. et al. (2012) Modelos de elevação para obtenção de atributos topográficos utilizados em mapeamento digital de solos, Pesquisa Agropecuária Brasileira (Online), 47, p.10,, accessed: 23.09.2013.

Riccomini, C., Sant’Anna, L. G., Ferrari, A. L. (2004) Evolução geológica do Rift Continental do sudeste do Brasil, V. Mantesso-Neto et al., eds. Geologia do Continente Sul-Americano: Evoluçao da obra de Fernando Flávio Marques de Almeida, Beca, 383–405.

Scheffer, F. et al. (2009) Lehrbuch der Bodenkunde (Sav Geowissenschaften) (German Edition), Spektrum Akademischer Verlag,, accessed: 30.09.2013.

UFF/FEC (2010) Planejamento Estratégico da Região Hidrográfica dos Rios Guapi-Macacu e Caceribu-Macacu, Niterói, Brazil.

Wesenberg, J., Seele, C. (2009) Floristic-structural composition and diversity of tree and woody understorey vegetation in the montane Atlantic Forest of the Serra dos Órgaos National Park, Teresopolis, RJ, Brazil, H. Gaese et al., eds. Biodiversity and land use systems in the fragmented Mata Atlântica of Rio de Janeiro, Göttingen, p. 46, 14_Wesenberg & Seele_259-280.pdf.

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