The Colorado River Estuary and Upper Gulf of California, Baja, Mexico
335
Temperature difference between surface and bottom water is less than
0.5 oc, with higher temperatures at the bottom during winter and higher
surface temperatures in the summer. Vertical temperature gradients may
be reversed during summer, with up to 4 °C lower temperatures at the surface than at 5 m depth, owing to the strong tidal turbulence. The horizontal
surface temperature gradient reverses at the beginning of spring and fall,
due to the annual cycle of atmospheric temperature. Surface temperature
increases from the southeast to the northwest in summer, while the opposite occurs in winter. Surface water temperatures range from 8.25 oc in
December to 32.58 oc in August west of Montague Island, while in deeper
waters of the southeast region seasonal temperatures range from 17 to
30.75 °C (Alvarez-Borrego et al. 1973).
The SE to NW transport along the Sonoran coast adds sediment from
the ocean into the estuarine basin, while N to S transport along the Baja
California coast removes sediments from the estuarine basin into the northern Gulf. The residual sediment transport pattern suggests a counterclockwise path of materials exchange between the Colorado River delta
and Upper Gulf of California (Carriquiry and Sanchez 1999). A net crossbasin sediment transport occurs in a NE-SW direction. The Sonoran sedimentary province is characterized by a heavy mineral suite of garnet and
zircon (GZ), whose sediment supply originates from the sandy sediments
of the Sonora Mesa deposits and Sonora Gran Desierto. The Baja Californian sedimentary province is characterized by a hornblende-epidote-piroxene (HEP) suite, whose sediments originate from earlier supplies of the
Colorado River. The intense reworking of the Colorado River delta has
dispersed HEP-rich sediments throughout the Upper Gulf and GZ-rich
sediments are invading areas previously dominated by HPE sediments
(Carriquiry and Sanchez 1999). The grain size of surface sediments at the
Sonora (3.7-2.8phi) and Baja California coast (6.8-4.7phi) increases
northward. However, while the organic matter (6 13 C and 6 15 N) in surface
sediments at the Sonora coast is mainly phytoplankton-derived, owing to
intrusion of oceanic waters, in sediments off Baja the contribution of
phytoplankton is low and bacterial biomass higher, probably due to anaerobic bottom waters (Aguifiiga-Garda 1999).
In general, turbidity increases towards the coasts, thus Sechii disk
depths are about 2m in deeper waters and between 0.5 and 1 m off Baja
California and Sonora, respectively. Due to sediment resuspension Sechii
depth around Montague Island is only a few centimeters. Seston values
range from 16 mg 1- 1 in October to 132 mg 1- 1 in May with 23-100 % as particulate organic matter (Garda-de-Ballesteros and Larroque 1976). Surface
concentrations of dissolved oxygen vary between 1.33 mll- 1 north of Montague Island in October to super-saturation(> 130 %) in southern and cen-
335
Temperature difference between surface and bottom water is less than
0.5 oc, with higher temperatures at the bottom during winter and higher
surface temperatures in the summer. Vertical temperature gradients may
be reversed during summer, with up to 4 °C lower temperatures at the surface than at 5 m depth, owing to the strong tidal turbulence. The horizontal
surface temperature gradient reverses at the beginning of spring and fall,
due to the annual cycle of atmospheric temperature. Surface temperature
increases from the southeast to the northwest in summer, while the opposite occurs in winter. Surface water temperatures range from 8.25 oc in
December to 32.58 oc in August west of Montague Island, while in deeper
waters of the southeast region seasonal temperatures range from 17 to
30.75 °C (Alvarez-Borrego et al. 1973).
The SE to NW transport along the Sonoran coast adds sediment from
the ocean into the estuarine basin, while N to S transport along the Baja
California coast removes sediments from the estuarine basin into the northern Gulf. The residual sediment transport pattern suggests a counterclockwise path of materials exchange between the Colorado River delta
and Upper Gulf of California (Carriquiry and Sanchez 1999). A net crossbasin sediment transport occurs in a NE-SW direction. The Sonoran sedimentary province is characterized by a heavy mineral suite of garnet and
zircon (GZ), whose sediment supply originates from the sandy sediments
of the Sonora Mesa deposits and Sonora Gran Desierto. The Baja Californian sedimentary province is characterized by a hornblende-epidote-piroxene (HEP) suite, whose sediments originate from earlier supplies of the
Colorado River. The intense reworking of the Colorado River delta has
dispersed HEP-rich sediments throughout the Upper Gulf and GZ-rich
sediments are invading areas previously dominated by HPE sediments
(Carriquiry and Sanchez 1999). The grain size of surface sediments at the
Sonora (3.7-2.8phi) and Baja California coast (6.8-4.7phi) increases
northward. However, while the organic matter (6 13 C and 6 15 N) in surface
sediments at the Sonora coast is mainly phytoplankton-derived, owing to
intrusion of oceanic waters, in sediments off Baja the contribution of
phytoplankton is low and bacterial biomass higher, probably due to anaerobic bottom waters (Aguifiiga-Garda 1999).
In general, turbidity increases towards the coasts, thus Sechii disk
depths are about 2m in deeper waters and between 0.5 and 1 m off Baja
California and Sonora, respectively. Due to sediment resuspension Sechii
depth around Montague Island is only a few centimeters. Seston values
range from 16 mg 1- 1 in October to 132 mg 1- 1 in May with 23-100 % as particulate organic matter (Garda-de-Ballesteros and Larroque 1976). Surface
concentrations of dissolved oxygen vary between 1.33 mll- 1 north of Montague Island in October to super-saturation(> 130 %) in southern and cen-