You may have noticed that the currents in your tide book don’t seem to be very accurate right now- particularly at Sherman Island and along Crissy field.
Why is that?
It’s a halocline!
As you might already know the Sierras have received record breaking snow falls this year. The snow pack is extremely deep as a result. Coupled with the warm spring conditions that we will have in the upcoming weeks, a tremendous amount of snow will melt. This will send torrents of fresh water into rivers, down into the delta and through the bay.
Salt water from the ocean and fresh water from the mountain snow melt have different densities. Salt water is more dense than fresh water. The fresh water can therefore ‘float’ upon the ocean’s salt water. Very surprisingly it takes quite a bit for the two layers to “mix” into a homogeneous state. A halocline develops as a result. During a rapid snow melt fresh water will flow down hill towards the Golden Gate. The tide still needs to rise and fall so the ocean water will “under cut” the fresh water drainage during flood tides. This is why it may continue to Ebb at the surface while the tide rises with in a flood tide. The following illustrations depict this quite well:
Ebb Tide: The salt water wedge and fresh water move in the same direction
Source: Encyclopedia Brittanica
Flood Tide: Salt water wedge undercuts the fresh water. The sea level rises, but the surface continues to Ebb.
Source: Encyclopedia Brittanica
To make things even more complicated, we need to take into account the complex shape of the bay. The example above is quite straight forward at a place like Sherman Island where the water flows one way, or the other, down a narrow channel. The central bay is far more complicated. For example, a very unique pattern developed on Sunday: The river of fresh water was flowing out of the north bay and all the way out of the gate. Meanwhile the tide was rising due to an undercutting salt water wedge. Keep in mind that the South bay does not have a fresh water sourse from the mountains feeding it. The water needs to get down there somehow in a rising tide. Some of that water may come from the north bay’s fresh water river. However much of it will still come from the ocean. We observed an intese upwelling of ocean water at Crissy Field just off of Anita Rock. A ripping flood developed and rolled along the marina green. This water wrapped around the city and flowed down to the south bay.
Crazy stuff!
So while the tides may rise and fall keep the halocline concept in mind if you are trying to figure out why the current in the central bay or up by Sherman Island seems to misbehave.
-Matthew Gregory
3-28-2011