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Hey! Nerds! It's been way too long since I've threaded about tidal wetland geomorphology. Let's talk king tides! https://twitter.com/SFEstuary/status/1339368210282496002
So, what the heck is a "king" tide? Well, the technical term is a "perigean spring" tide. It's when a new/full moon is in alignment with Earth & sun, and is closest to Earth. In SF Bay, they happen a few times in the winter (during the day), and a few times in summer (at night).
In the tidal marsh world, we generally talk about the elevations of marshes relative to what's called "tidal datums" - the elevations of statistically meaningful tidal averages. SF Bay has what's called "mixed diurnal" tides - every day (technically, every 24.8 hours), >
the Bay has two high and low tides of unequal value. The datum MHW, or Mean High Water, is the average elevation of all the high tides over a tidal epoch (19 yrs). Mean Higher High Water, or MHHW, is the average of the all the higher high tides.
Most mature tidal marshes in the Bay (of which there are very few left!) have marsh plain elevations *above* MHHW. Which means that they don't actually get inundated all that frequently, or for very long, by typical tides.
This isn't the case when king tides occur. During king tides, high marsh plains are inundated at greater depths, and for a much longer period of time, than they are during typical spring (higher) tides.
Now, a basic tenet of SF Bay tidal wetland restoration is that the lower in elevation a site is when it's restored to tidal action, the faster it initially accretes sediment (assuming adequate sediment supply), because the site basically acts as a decanter (yes, like wine!).
A muddy flood tide washes in, full of sediment from the Bay and local watersheds, and at/around slack tide (that period when the tide is neither flooding in nor ebbing out), the sediment settles to the bottom of the site.
Then, on the subsequent ebb tide, clearer water flows from the restoration site back into the Bay. The lower the site is, the larger the volume of tidal water that flows in and out (called "tidal prism"), so more sediment drops out out of the water column and onto the site.
This is why some Bay restoration sites in areas with abundant suspended sediment, like the Island Ponds in the Lower South Bay, were able to gain so much elevation so quickly after breaching, and get high enough to support veg. (L: shortly after breaching in 2006, R: late 2019)
So our basic conceptual model implies that as a tidal wetland restoration site gains elevation, it gets inundated by a smaller tidal prism less frequently, so the rate of accretion due to tidal deposition of suspended sediment generally slows down.
Ergo, you would think that marshes that are already high and mature (like China Camp) depend on events like king tides to help them accrete sediment and gain elevation, right? Because it's one of the few times these high marsh plains get inundated?
Well, surprise surprise: turns out it's way more freaking complicated than that!!!
Marshes don't just accrete sediment due to tidal settling of suspended sediment - they also accrete sediment due to waves that pull unconsolidated sediment from adjacent mudflats and pile it onto the bayward edge of the marsh.
And tidal inundation, wave action, and the availability of sediment in Bay waters that can accrete on the marsh plain (from watersheds and adjacent mudflats) vary between seasons, between years, and between tidal cycles (spring vs. neap). It's freaking COMPLICATED, yo.
I'd argue that the scientist with the most comprehensive and nuanced understanding of these dynamics is Dr. Jessica Lacy, from @USGS. Every time I'm at a conference where she's giving a talk, I make sure to listen, because I always learn something new. https://www.usgs.gov/staff-profiles/jessie-lacy?qt-staff_profile_science_products=0#qt-staff_profile_science_products
Jessie has done a lot of research on geomorphic processes driven by tides and waves at China Camp marsh - the same marsh in that photo in the OP. (For more on king tide flooding of San Pedro Road through China Camp, see https://twitter.com/ChristinaToms/status/1205583947284959232?s=20)
In this paper co-written with Dr. Matt Ferner and @drjohncallaway, she demonstrated how king tide inundation at China Camp actually resulted in a net *loss* of sediment from the marsh system, due to large scouring ebb flows in the tidal channels: https://pubs.er.usgs.gov/publication/70197482
Now, that doesn't necessarily mean that the marsh plain eroded (lost elevation), but it reduces the amount of sediment available for deposition on the marsh plain.
That study and follow-up research indicated that at China Camp, a lot of the deposition on the marsh plain is dominated by waves, not tides. This is especially true for the more consistent moderate summer waves than the less consistent larger winter waves: https://pubs.er.usgs.gov/publication/70208112
Now, what's true at China Camp may not hold true for all the Bay's marshes. China Camp's mature marsh plain (dominated by pickleweed) is fronted by a transitional ramp of lower marsh (dominated by cordgrass). Not all marshes in the Bay are like this.
Many are instead fronted by steep, unvegetated scarps. So Jessie and @drKarenThorne are implementing a new study at Whale's Tail marsh in the South Bay that will investigate the spatial and temporal differences in marsh accretion driven by both tides and waves at a scarped marsh.
Sometimes scarps are indicative of erosive conditions. Sometimes not. I'm really looking forward to seeing the data from their study, which will cover a king tide period!
So, what have we learned? Tidal wetland geomorphology - the relationships between tides, waves, sediment, and vegetation - is complicated! King tides may bring a lot of material into a marsh, but they can scour a lot of it out, too, through tidal channels.
So when you're observing king tides (and hopefully uploading your observations to @CA_king_tides), you're not just seeing a preview of sea level rise - you're also observing a unique set of physical circumstances that can alter tidal wetland geomorphology!
Thanks for listening, and please feel free to drop any questions below!
