Until 1930 the Upper Gulf of California was an estuarine system into which 20x 10 9 m3 ·yr-1 of fresh water, and 180x 10 6 tons·yr-1 of sediment were supplied by the Colorado River. Damming of the river and the high evaporation turned the Upper Gulf into an inverse estuary with high turbidity, attributed to tidal erosion and resuspension of bed deposits in the river delta. The origin and distribution of suspended particulate matter has been scarcely studied, mostly through indirect estimates of turbidity levels. The tide has been assumed to control the amount of sediment in suspension, but detailed observations
of suspended matter concentration and tidal forcing were unavailable prior to this study.
In August 1997 an intense survey was made across the Upper Gulf in which current meters, transmissometers, optical backscatter sensors and an acoustic Doppler current profiler were deployed during one fortnightly cycle. Water column data using a profiling CTD and optical sensors were obtained during spring and neap tides at fixed stations, as well as in a gulf-wide survey. During a preliminary two-day survey in June 1996 two current meters were deployed off the western coast of the Upper Gulf. Observations using CTD and an optical sensor were made during spring and neap tides. The surveys have
supplied the first detailed profiles of SPM concentration, current velocity and
hydrography, under contrasting tidal conditions, and continuous records of near-bed and mid-water SPM concentration during a fortnightly cycle.
A tidal logarithmic bottom boundary layer was observed in a nearly homogeneous water column over most of the fortnightly cycle, except during short periods near slack water, and during neap tides. Observed SPM concentration varied from 10 -2 kg·m-3 to 10-1 kg·m-3 near the sea bed, and decreased upwards to ~0.005 kg·m-3 near the sea surface. The main
control of SPM concentration was tidal resuspension, as shown by the quarter-diurnal, fortnightly-modulated peaks in the SPM time series. In contrast, during neap tides the interaction between slow tidal currents and near-bed gravity current events induced weak resuspension peaks at the semidiurnal frequency. Critical bed stress for erosion was in general 0.6 Pa - 0. 7 Pa during spring tides and one order of magnitude smaller during neap tides. A sediment deposition threshold was not evident in the data. The bed stress SPM concentration time series follows a continuous deposition model better than a mutually exclusive erosion-deposition model for cohesive sediments. A simple erosion deposition point model overestimated the observed SPM concentration due to an excess
of erosion over deposition, a discrepancy ascribed to an underestimation of the prescribed single-class particle settling velocity. Based on observed current velocity and calibrated acoustic backscatter signal levels from the ADP, the maximum horizontal SPM fluxes were ±30 g·m-2-s-1 during spring tides and
-6 g·m-2-s-1 during neaps. A residual SPM flux of- 2.5 g·m-2·s-1 out of the Upper Gulf was obtained in neap tides due to the near-bed flow of denser, warm and saline water during gravity current events. This result shows that the
inverse estuarine nature of the Upper Gulf is a key factor influencing the transport of suspended sediment at least during summer.