The Gulf of California comprises a large evaporation basin which, at its southern end, is in open communication with the Pacific Ocean. Surface temperatures range between 14°C and 21°C in February, and between 28°C and 31°C in August. Surface salinities fluctuate mostly between 35.0^pmil and 35.8^pmil and change little with season. The amplitude of the mean annual sea-level oscillation observed at tide gage stations is about 15 cm, and agrees closely with that of the steric sea-level oscillation above 500 m. The mean tidal range increases from about 1 m at the Gulf entrance, to 7 m near the mouth of the Colorado River. This river, which is rich in sulfate, sodium, and calcium, has a mean annual discharge rate of 120 m³/sec. Northwesterly winds i winter drive the surface water southward and cause extensive upwelling along the east coast and in the lee of Isla Tiburon; southeasterly winds in summer move the surface water northwestward and give rise to some upwelling along the Baja California coast. The order of magnitude of the upwelling speed is 3 m per day. In the northern part of the Gulf, winter cooling may lead to convective overturn near the coast that may extend downward to about 100 m. The subsurface circulation in the Gulf is rather complicated. The salinity distribution suggests that there is inflow of low-salinity water along the east coast and outflow of high-salinity water along the west coast, at depths between 50 m and 100 m. Below the bottom of the thermocline, the water in the Gulf is essentially the same as else here off southern Mexico; it is characterized by a salinity minimum of less than 34.60^pmil between 400 m and 800 m, and by an oxygen minimum of less than 0.2 ml/L between 200 m and 800 m. In some instances, the amount of dissolved oxygen was below the titration accuracy (0.05 ml/L). At depths between 2,500 and 3,000 m there is a temperature minimum of slightly less than 1.85°C; from there to the bottom, the temperature increases again, mainly as a result of adiabatic compression. Unusual hydrographic conditions, not found elsewhere in the eastern Pacific, are encountered in Ballenas Channel, which is completely isolated from the rest of the Gulf below 250 m. Here the bottom (1,500 m) temperatures are about 11°C, or about 8°C higher than at comparable depths outside the chann l.