Water level in marsh pond
September 12, 2018 9:12 AM   Subscribe

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.
posted by salt grass to Science & Nature (7 answers total)
It probably gets inundated in a high-high tide and not on the low-highs. The height of the tide changes throughout the month and seasonally. The variation is also called spring vs neap or king tides or negative tides, you can look it up and see if it correlates but I bet it does.

Then it'll fill up with rain or overland flow of freshwater too obviously.
posted by fshgrl at 9:36 AM on September 12, 2018 [2 favorites]

there may be a buffering action from groundwater that delays / smooths the immediate effect of the tides.
posted by zippy at 9:57 AM on September 12, 2018

fshgrl: thank you for that suggestion--I'll make notes next time I see a fluctuation, and then look up those extreme tides to see if they correlate.
posted by salt grass at 10:32 AM on September 12, 2018

Directly across the highway from the pond is the East Bay Municipal Utility District Wastewater Treatment plant, and immediately adjacent to the highway is what appears to be a holding tank, chlorination equipment, and a pumphouse.

All sewage treatment plants around the SF Bay dispose of their treated water into the Bay, and that pond you point out is in just the right place to be part of the outflow from that plant, or at least to be affected by the outflow from the plant.

I would actually expect the effluent to be dumped into the channel that is a couple of hundred feet to the north and east of the pond you speak of. There are a pair of suggestive (to me) manhole covers that seem to indicate the path of that pipeline, but there could be significant leakage from an old pipe like that, which could be contributing to your pond.
posted by the Real Dan at 12:29 AM on September 13, 2018 [1 favorite]

I wondered about that, too, the Real Dan. I may write to EBMUD and ask; they've been responsive in the past to other random questions.
posted by salt grass at 7:03 AM on September 13, 2018

If you wanted to know the tide levels in the bay, the nearest NOAA guage is in Richmond which should give you historical levels as well as predictions.
posted by domo at 10:52 AM on September 13, 2018 [1 favorite]

The next tour appears to be on October 6.

EBMUD wastewater treatment plant tour signup
posted by the Real Dan at 12:46 PM on September 16, 2018

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