Baia de Guanabara, Rio de Janeiro, Brazil
109
to north. This changed physiography evolved during the Quaternary, as the
drainage patterns were adjusted during one or more lower stands of sea
level. The bay was further modified by the marine transgression during the
Holocene. Present-day sedimentary remnants of the Tertiary are the Macacu and the pre-Macacu formations northeast of the bay (Meis and Amador
1972, 1977). Coring in the bay only revealed late Pleistocene deposits and
braided fluvial sands related to the Caceribu formation (Amador 1980b,
1993). During the Holocene marine transgression 5,000 years BP, relative
sea level extended 4 m above the present sea level and the bay was then
more than twice the size (800 km 2 ) of the present bay. Elevated paleobeaches and marine terraces ( +4 m) commonly occur around the bay
(Amador 1974; Amador and Ponzi 1974).
Flooding of the drainage basin during the Holocene transgression was
responsible for extensive mud deposition on top of fluvial Pleistocene
sands. As evidenced by high-resolution seismic profiling, mud deposits
attain a thickness of more than 10m near the Rio-Niter6i bridge. Along the
interior margin of the bay, the distribution of mangrove wetlands conforms with the southwest-northeast orientation of the basin and the orientation of the dominant Rio Guapimirim and Rio Macacu catchment basins.
During the last half-century, Rio Macacu and other meandering rivers
have been channelized to reclaim alluvial lands for urban growth. The
lengths of the rivers have in several instances been reduced to 30% of their
previous lengths. This caused higher flow velocities, local scouring, and
increased sediment load. River channelization combined with catchment
deforestation increased sedimentation to Baia de Guanabara. The mean
sedimentation rate used to be 0.24 m century- 1 during the period 18491922, but increased to 0.81 m century- 1 from 1938 to 1962 (Amador 1980a).
Recent sedimentation rates vary spatially from 0.57 to 4.50 m century- 1
within the bay (Amador 1992).
8.3 Bathymetry and Bottom Sediment
Baia de Guanabara has a complex bathymetry with a relatively flat central
channel. The channel is 400 m wide, stretches from the mouth more than
5 km into the bay, and is defined by the 30-m isobath. The deepest point of
the bay measures 58 m and is located within this channel. Numerous outcrops of the crystalline basement rock exist within the bay, both above
and below the water level, and pose a risk to navigation. The central channel widens to 900 m inland of the main choking point, and the depth
decreases to 20m. North of the Rio de Janeiro-Niter6i bridge, the channel
109
to north. This changed physiography evolved during the Quaternary, as the
drainage patterns were adjusted during one or more lower stands of sea
level. The bay was further modified by the marine transgression during the
Holocene. Present-day sedimentary remnants of the Tertiary are the Macacu and the pre-Macacu formations northeast of the bay (Meis and Amador
1972, 1977). Coring in the bay only revealed late Pleistocene deposits and
braided fluvial sands related to the Caceribu formation (Amador 1980b,
1993). During the Holocene marine transgression 5,000 years BP, relative
sea level extended 4 m above the present sea level and the bay was then
more than twice the size (800 km 2 ) of the present bay. Elevated paleobeaches and marine terraces ( +4 m) commonly occur around the bay
(Amador 1974; Amador and Ponzi 1974).
Flooding of the drainage basin during the Holocene transgression was
responsible for extensive mud deposition on top of fluvial Pleistocene
sands. As evidenced by high-resolution seismic profiling, mud deposits
attain a thickness of more than 10m near the Rio-Niter6i bridge. Along the
interior margin of the bay, the distribution of mangrove wetlands conforms with the southwest-northeast orientation of the basin and the orientation of the dominant Rio Guapimirim and Rio Macacu catchment basins.
During the last half-century, Rio Macacu and other meandering rivers
have been channelized to reclaim alluvial lands for urban growth. The
lengths of the rivers have in several instances been reduced to 30% of their
previous lengths. This caused higher flow velocities, local scouring, and
increased sediment load. River channelization combined with catchment
deforestation increased sedimentation to Baia de Guanabara. The mean
sedimentation rate used to be 0.24 m century- 1 during the period 18491922, but increased to 0.81 m century- 1 from 1938 to 1962 (Amador 1980a).
Recent sedimentation rates vary spatially from 0.57 to 4.50 m century- 1
within the bay (Amador 1992).
8.3 Bathymetry and Bottom Sediment
Baia de Guanabara has a complex bathymetry with a relatively flat central
channel. The channel is 400 m wide, stretches from the mouth more than
5 km into the bay, and is defined by the 30-m isobath. The deepest point of
the bay measures 58 m and is located within this channel. Numerous outcrops of the crystalline basement rock exist within the bay, both above
and below the water level, and pose a risk to navigation. The central channel widens to 900 m inland of the main choking point, and the depth
decreases to 20m. North of the Rio de Janeiro-Niter6i bridge, the channel