In the Florida Keys, the stream tends to be pushed offshore and a counter current between the reef and the Gulf Stream often results. The counter-current flows in a westerly direction along the south edge of the reef. As with tides everywhere in the world, the flow is governed by the gravitational pull of the sun and moon. The most extreme tides, spring tides, occur when the sun and moon are lined up and act together. The smallest rise and fall, neap tides, occur when the sun and moon are at right angles to each other, exerting less combined gravitational power.
Tides are most extreme when the moon is in perigee or closest to the earth.
Extreme tides are called perigean spring tides. During times of a perigean low spring tide, boaters can run aground in areas that ordinarily offer enough depth to operate. Semi-diurnal tides operate on a time period of approximately six hours and 20 minutes between high and low water. This makes the time of total movement of water from one high tide to the next approximately 12 hours and 40 minutes, and accounts for the continual change in tide times. The old standby is the government publication known as the Tide Tables.