Tuesday, November 08, 2011

Tidal wetland sediment accretion might keep up with sea level rise in one location. Maybe.

I attended our annual Santa Clara County Creeks Conference last Saturday, with an even better than usual program that included a panel on tidal wetlands restoration in South San Francisco Bay, where we're bringing back 16,000 acres of tidal wetlands from former saltponds (will post a video link when it's online).


The restoration has barely begun, but the land that sank after being separated from tidal flows has gained sediment rapidly, something that's necessary to create a complex environment of open water, partially submerged, and emergent tidal environments. While it's slowed more after the first few years that individual ponds have been opened to the the tides, they're still adding sediment, two inches annually, far more than the worst projections for sea level rise.

So, good for us. Except that California is a geologically young area with lots of gradients, erosion, and sediment flow. Our particular part of San Francisco Bay might also disproportionately benefit from the "backwash" of sediment from the rest of the Bay.

Our tidal wetlands can keep up where they are, for now, but whether that will work in other places is less clear. Still, it's one small piece of good news that demonstrates the value of restoring tidal wetlands, which have been lost to a far greater extent in the US than even freshwater wetlands have.

17 comments:

Fixed Carbon said...

Any mention of the invasive hybrid Spartina phenomenon? The large, fast growing Spartina that was created in SFB when native S. foliosa crossed with introduced S. alterniflora can capture a lot of sediment. The result can be bad (habitat lost to migratory shore birds, storm water runoff blockage, need to dredge channels, marsh restoration for most species except hybrid Spartina) and good (endangered clapper rail thrives in hybrid Spartina, the plant can slow tidal rise and protect levees at very high water: the coming times of higher sea levels).The state has spent lots over the last decade killing the hybrids. We might think about encouraging them in some areas in SFB vulnerable to very high water.

Jim Bouldin said...

I'd give my left arm for a chance to see the Bay and the Delta prior to 1850.
Good job Brian!

Anonymous said...

Dr. Jay Cadbury, phd.

I'd like to correct an obvious error in this paper. There is no mention of global warming. Obviously, this is a big mistake because everything is a result of global warming.

J Bowers said...

Oi, Cadbury, stop waffling on about global warming. Where's my pizza?!

David B. Benson said...

Brian --- Good!

Steve Bloom said...

Jeez, I'm such a naysayer about this. I would expect sediment compression to have created low spots that would preferentially fill with new sediment once runoff was restored (albeit only temporarily, as noted), but depending on backfill for long-term survival still seems like wishful thinking. Do you have anything hard on that, Brian?

The nightmare scenario for me is that the levees get built/raised, eliminating the path for upland retreat (a demonstrated pathway for wetland survival, BTW), and that the broader public gets to foot the bulk of the bill (as contrasted to property owners at risk of flooding) based on a scientifically shaky assurance of long-term wetland survival.

Brian said...

Fixed Carbon - yep, they talked about Spartina. They want to keep out the invasive stuff, but what they've got is all native, apparently.

Jim - you can't see it, but SFEI has a great historical ecology mapping program for SF Bay Area: http://www.sfei.org/he

Steve - I don't know the mechanism for the lost elevation in salt ponds (do you? I assumed drying and collapse of clays, not mechanical compression). The salt ponds are extremely flat, but as sediment returns it recreates a fractal-like system of channels. As for data, the backfill has been very successful on the ponds to date that have been opened up. Whether that will continue for the long term is another question. Considering that we're only opening up a few hundred acres at a time, each one of those ponds could correspond to the local low point you talked about that get preferentially filled. Whether the many tens of thousands of tidal wetlands in the Bay Area could accrete at the same rate if they're all competing for the sediment is another question.

As for tidal wetlands retreating upland, just go tell Google to abandon their corporate headquarters. Or tell me and my hundred thousand plus neighbors to move, I suppose - I'm only two miles from the Bay. I just don't see that happening.

Fixed Carbon said...

Brian: I know that claims of pure S. foliosa in south SFB are bogus. My lab has worked on this problem for 20 years. We discovered the hybrid and developed the molecular techniques for following the invasion. What will be really interesting is what happens in those restoration sites to birds (different birds will respond differently), to sediment dynamics (and its lack!), and to the change over time of hybrid Spartina (which will feedback to birds and to sediment dynamics).

Jim Bouldin said...

How in the hell did I miss that SFEI site?? Excellent link, many thanks Brian.
I like driving around/through the flats of San Pablo Bay. That gives me at least some idea, at least in the North Bay, sans the massive development you have down at your end.

Jim Bouldin said...

Are those foliosa X alterniflora hybrids self fertile now Fixed Carbon? If so, any data/reports/guesses on how that's changing the population dynamics?

Fixed Carbon said...

Jim: Here you go.
We have four or five new mss within the year that will show that south SFB is loaded with hybrid spartina.
http://spartinasfb.blogspot.com/
http://chinaspartina.blogspot.com/

Hank Roberts said...

> spartina
Very interesting stuff; any notion of delaying the most aggressive of the invasives long enough for selection to favor hybrids that fit better into the local ecology?

Seems delay is the best we can hope for when a new invader comes in, so the invader doesn't become a monoculture that collapses.

I'm poking at this with a forest fire restoration at about 5000' a few hours north -- where bromus tectorum (cheatgrass) is an aggressive post-fire invader. Current hope -- spraying sugar water after burning so the local microbes soak up the highly available minerals during the first winter, and in the spring the aggressive shallow rooted invasives don't get their post-fire benefit.

Hoping that the natives have a little longer to reestablish.

https://www.google.com/search?q=cheat+brome+fire+carbon+sugar

Brian said...

Fixed C - well, that's interesting about it being hybrid. I'll check the video when it's out, but they seemed pretty confident. Note they're talking about the three restored ponds, not the entire South Bay.

Hank - FC probably can give you a better answer, but I know they say they're removing it aggressively, and just hoping to keep it under control. Also, I thought cheatgrass was a desert problem.

Jim Bouldin said...

Cheatgrass is widespread over the mountain and intermountain west, all the way to the coast. Especially the dryer or colder parts. And it's altering ecosystem function in a potentially big way (via fire spread rates), just as invasive cordgrass may.

Fixed Carbon said...

Brian: The issue is what happens when the restored ponds are opened. Several ponds have been opened in the last 15 years. Each has been invaded by hybrid Spartina, the seeds of which float on the tide. Because the ISP has killed so much hybrid Spartina, the invasion of these post modern ponds will proceed more slowly.

Anonymous said...

We all should be so lucky. What barriers hold the marshes in place, separate from the Bay? Our (Texas) usual are barrier islands which migrate inland and fall apart (due to a lack of sand supply) making sediment accretion rates in tidal marshes moot. Once unprotected from major wave action they wash away. Likewise many marshes in Texas and Louisiana easily keep up with sea level rise through organic accretion. Again, this is moot as the structures which separate them from the ocean are falling apart with even the modest amount of sea level rise (mostly subsidence at this point) we've experienced so far.

We've done a lot of marsh creation with dredged sediments in Texas, but the barriers used to hold the mud in place are very expensive and require lots of maintenance. Here's the web site. Please excuse the fluff. This was done by an advertising consultant for the Port of Houston (betterbay.org). Making tidal marshes in a micro-tidal environment where the mud must be pumped in.

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