Water level in marsh pond

Post date: 2018-09-13 05:12:53

There's a pond in the tidal marsh near Emeryville that I see most weekdays, and my question is about that pond's pattern of water levels. Here's an aerial image of it.

The level of San Francisco Bay's tide is easily visible in this area of the bay; the channels in the marsh vary from "quite full" to "lots of bank exposed".

The pond also sees water level changes, but much less frequently than the tidal cycles--the low water marks do not appear linked to low tide, nor to anything like heatwaves or weather generally. At irregular intervals, the water level of the pond gets quite low, and then days later it appears full again.

What makes the level of the pond vary? The rate of change seems too rapid for evaporation to be the main mover. That said, I have no grasp at all of how tidal marshes "work", hydrologically.

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