BaíadeSepetibabasins

The basins of the Baía de Sepetiba, located in the western part of the city of Rio de Janeiro, represent a complex articulation between the small landforms of the isolated mountain ranges and the low altitudes of the western slope of the Maciço da Pedra Branca, starting from the Serra de Guaratiba. The most striking landform expression of the region is undoubtedly the extensive coastal plains bordering the bay. The river geography of this region is shaped mainly by the western slope of the Maciço da Pedra Branca and by the southern and southwestern slopes of the Maciço de Gericinó-Mendanha, which together give rise to rivers of relatively short course, though with a torrential regime in their upper sections and low gradients across the alluvial plains, until they reach the mangroves and estuaries of the Baía de Sepetiba.

Image description: This is a photograph of a mountainous landscape. From left to right, we see a series of hills covered in dense green vegetation. In the center, there is a more open area with distant buildings, possibly a city. On the right, the mountains continue, with some areas of sparser vegetation. The sky is clear and blue, with few clouds.

Valley of the Riacho Virgem Maria, which forms the Rio Lameirão, flowing down from Pico da Pedra Branca toward the north, then turning West toward the Baía de Sepetiba, making up the drainages of the Rio Cabuçu-Piraquê.

Image description: This is a photograph of a waterfall amid a dense forest. On the left side, we see large rocks and tall trees with vibrant green foliage. In the center, water falls from the waterfall, forming a small, calm pool. On the right, more rocks and dense vegetation complete the scene. The pool reflects the sunlight, creating patterns of reflections on the water's surface.

Lago dos Encantos, on the Riacho Virgem Maria, which forms the Rio Lameirão, one of the geotourism sites of the Quilombo Dona Bilina, on the northern slope of the Maciço da Pedra Branca.

Within the boundaries of the municipality of Rio de Janeiro, the rivers Guandu-Mirim, Cação Vermelho, Cabuçu, Piraquê and Portinho stand out. All of them have their headwaters in the foothills of the massifs, on land made up of crystalline rocks and hillside deposits, generally with rocky boulders, giving rise to a hydrography marked by narrow valleys, waterfalls and steep slopes. However, they are soon connected in sequence by the extensive sedimentary lowland where the neighborhoods of Santíssimo, Paciência, Inhoaíba and Campo Grande are located, all the way to Santa Cruz, already on the coast of the Baía de Sepetiba. Also along the Bay's coast are the neighborhoods of Sepetiba and Guaratiba.

The Guandu-Mirim river, a tributary of the great Rio Guandu, rises on the slopes of the Serra do Mendanha, within the Parque Estadual do Mendanha, and runs parallel to Avenida Brasil to the neighborhood of Canto do Rio, where it meets its main river right at the municipal boundary. In its upper stretch, it forms beautiful waterfalls and cascades, nestled in narrow valleys of the Serra do Mendanha and shaded by the abundant vegetation of the Atlantic Forest. In its lower portions, however, the river spreads out across flat areas that have historically been used for agriculture along the Estrada do Guandu do Sena.

The Cabuçu river has its headwaters on the northwestern slope of the Maciço da Pedra Branca and collects water from an extensive basin stretching from the foot of the Pedra Branca peak in the central portion of the massif. The drainage is set within weakness zones of the geological substrate, rich in granites but penetrated by intrusions of more fragile rocks that allow the dissection work of the drainages. This is the case of the Lameirão river, which at its headwaters holds the old reservoir of the Fábrica de Bangu, still with the ruins of its aqueduct. The river cuts through areas occupied by remaining quilombola communities and some agricultural practice, until it reaches the urbanized lowlands toward the Guaratiba neighborhood, south of the Serra de Inhoaíba, parallel to Estrada do Magarça. Before reaching the estuarine areas, the natural lowland system held marshes and wetlands that were paved over by various subdivisions planned on their floodplains. The result of this urbanization could be nothing other than the recurrence of flooding events and flood victims. It then becomes the Piraquê river, draining the apicum and mangrove areas up to Pedra de Guaratiba.

Image description: This is a photograph of a river flowing through a dense forest. On the left side, we see large, slippery rocks covered in green moss. As we move to the right, the river becomes wider and shallower. The center of the image highlights a small waterfall, with water flowing gently over gray stones. On the right, there is a larger rock formation, with vegetation hanging over it. The background is full of green trees, creating a tranquil, natural atmosphere.

Corredeira do Lajedo, on the Virgem Maria Stream, which forms the Lameirão River, another geotouristic spot of the Quilombo dona Bilina, on the northern slope of the Maciço da Pedra Branca.

The Rio Portinho, in turn, flows down short slopes of the Serra de Guaratiba, quickly reaching areas of sandy, fluvio-marine and then marine plains, full of salt flats and mangroves protected by the Reserva Biológica de Guaratiba. A complex labyrinth of channels tangled among the mangrove fringes forms an exuberant landscape that recalls what the other areas of tidal creeks and estuaries at the opposite end of the city, in the former Saco de São Diogo and Saco de Inhaúma, in Baía de Guanabara, must once have been like.

Image description: This is an aerial photograph of a coastal landscape. On the left side, we see an extensive green area densely covered with vegetation, stretching to the water's edge. As we move toward the center of the image, we notice several areas of brackish water, in shades of gray and beige, interspersed with green areas. On the right, the horizon aligns with the sea, where the blue of the ocean contrasts with the clear, blue sky. On the distant horizon, there is a landform that could be an island or a peninsula. The sky is clear, with a few scattered clouds.

Meanders and wetlands of the Piracão River preserved within the Guaratiba Biological Reserve (REBIO – ICMBio), recalling what the lowland and river-mouth environments of our rivers were like before the channeling and landfills.

Image description: This is an aerial photograph of a city. From left to right, we see densely urbanized areas with many buildings. In the center, there are green areas and open spaces, contrasting with the urban density. As we move to the right, the city extends toward distant hills, covered in green vegetation. The sky is blue with some white clouds. There is no visible text in the image.

The Rio Cação Vermelho, in the Campo Grande lowlands, between the Serra de Paciência and Inhoaíba, even at a bend, is found channeled between concrete walls.

Image description: This is an aerial photograph of a city. From the left side toward the center, we see green areas and scattered buildings. In the center, there is an open field with some distant structures. On the right, the city extends to the horizon, where mountains covered by clouds stand out. The sky is light blue with some white clouds.

Mouth of the Rio Cação Vermelho, along with the mouth of the Rio Guandu and several canals, all straightened and parallel, built with the intention of “drying up” the swampy lowlands at the back of the Baía de Sepetiba.

The Cação Vermelho river represents a classic lowland coastal river, moving slowly through poorly drained lowlands toward the Baía de Sepetiba. Today its waters are channeled through a network of straight, parallel canals, built with the intention of draining the entire Santa Cruz lowland. In this stretch, extensive organic soils buried by urbanization bear witness to the organic nature of the coastal lowland soils, once full of marshes and floodplains. Once again, the DNOS (National Department of Sanitation Works), in the early part of the century, did not let this area slip by, creating sub-parallel lines from the Guandu river, the São Fernando canal, the Guandu canal, the Irrigação canal, the Dom Pedro canal, all the way to the Itá canal and the Ponte Branca canal. Works upon works in a frustrated attempt to “dry out” the area of a bay-bottom estuarine complex. Sedimentary coastal plains, made up of recent, poorly drained deposits, function as buffer zones for floods and tides; the plain is marked by wetlands, floodplains, and mangroves, which historically played a fundamental ecological role, acting as natural filters and nurseries for marine life in the Baía de Sepetiba.

In the stretch where it crosses the municipality of Rio de Janeiro, the Guandu River plays a crucial role in the hydrographic geography of the west zone. After receiving waters from several tributaries originating in the Maciço de Gericinó-Mendanha, the river skirts the Serra do Mendanha and flows through areas of Santa Cruz, amid a fluviomarine plain of recent sedimentation. Although its source lies outside the municipal boundaries, its lower course directly touches Rio de Janeiro's territory before flowing into the Baía de Sepetiba, contributing an important volume of water responsible for supplying the Metropolitan Region. In its surroundings, remnants of mangroves and wetlands can still be observed, resisting growing urban and industrial pressure. Unlike the basins of Baía de Guanabara, where urbanization took hold earlier, the basins of Sepetiba remained under relative preservation for longer, which allowed for the maintenance of a river dynamic closer to its natural regimes — although, in recent decades, urban and industrial expansion has significantly affected its water quality.