Inverse-estuarine Features of the Upper Gulf of California

The Upper Gulf of California is the shallow (depth <30 m), tidal area at the head of the Gulf of California. It is an inverse estuary, due to the high evaporation rate (E 1•1 m year−1) and almost nil freshwater input from rainfall and the Colorado River. Historical and recent hydrographic data show that the area is almost vertically well-mixed throughout the year, that the horizontal distribution of properties follows the bathymetry, and that the hydrography has a strong annual modulation. As in other negative estuaries, the year-round salinity increase toward the head causes the density to do likewise, despite the seasonally reversing temperature gradient. The pressure gradient thus formed leads to water-mass formation and gravity currents (speed 0•1 ms−1), both in winter and in summer. In winter, the high salinity water sinks beyond 200 m, while in summer it only reaches a depth of 20–30 m. The gravity currents appear to be modulated by the fortnightly tidal cycle, with events in neap tides. This phenomenon causes the presence, at least during neap tides, of slight stratification (Δσt≈−0•2).