Fish in the Bay – 2 December UC Davis Trawl – Part 2: King Tide Weekend; The Tide

Folks, … something a little different this time.

I keep meaning to discuss the most conspicuous feature you see on any given boat outing in Lower South Bay.  The tide itself literally jumps out as you cruise the sloughs.  This is partly because the Bay acts like a giant funnel that amplifies tidal height in the southern end.  Pacific tides at the Golden Gate range 3 feet from average high to low, but tides increase to double as they move down the Bay.  The range of tide at the Coyote Creek station is 7 feet!  And, extreme “Spring Tides” we see in December and January range add a few more feet.

Editor’s note:  Senior process engineer, Issayas Lemma, later corrected me regarding the above statement.  Any large gradient of decreasing ocean or lake depth will amplify tidal height.  The amplification has to do with decreasing depth, not the geographical shape of our Bay.

It can be kind of like the tidal wave scene in the 2014 Matthew McConaughey movie “Interstellar.”  You might be drifting by a vast mud flat at low tide one minute, and before you know it, you are in the middle of a lake.

Tide swings in Lower South Bay are not quite as dramatic as the movie, but I will show some examples from recent boat trips:

Alviso Launch – 2 Dec, 8:47 AM.   We had a full Moon on December 3rd.  This means that the 2-3 December weekend experienced the month’s higher-high and lower-low “Spring Tides.”  Plus, Earth was nearing perihelion (closest orbital approach to the Sun) in early January which adds to tidal range. Above, as Dr. Hobbs sacrificed the sprinkle donut to the Sea Gods, tide was already high and rising.  Even three hours before high tide, nearby marsh vegetation was nearly submerged.

Pond A6 – 2 Dec 2017, 10:00 AM:  90 minutes prior to high tide.  A little over an hour later, I snapped this shot of Pond A6 looking into the breach.  You can see the pond is flooded, and tall marsh vegetation (mainly Spartina) is totally submerged.

Pond A6 – 2 Dec 2017, 3:56 PM: falling tide.   This was close to the same view into Pond A6 six hours later.  (If you look closely, you can see two humps in the right-side remnant levee in both photos to give a sense of the water height change.)  Bear in mind that these two photos show only a portion of full tidal range: above shot was an hour before higher-high tide, and picture below is still at least three hours before lower-low tide – it gets lower!

Example #2Outfall Bridge – 4 Dec2017, 12:30 PM: High Tide.  I observed higher-high tide at our Bridge over our outfall just after noon the following Monday.  Ryan Mayfield was standing in the middle of the bridge with his feet barely a foot over water level.

Outfall Bridge – 4 Dec 2017, 5:00 PM = Five foot drop in 4.5 hours!  Just four-and-a-half hours later, most of the water was gone.  It was still more than 3 hours away from low tide, but Sun was setting, and I could not photograph this scene at night.

Bugs, birds and fish evolve and adapt to the tide.  Some of our native California species find a niche for themselves in these extreme tide swings: perhaps a niche that puts some invasive species, like Corbula clams, at a competitive disadvantage.

Example #3: Pond A19 – 10 Apr 2016, 10:22 AM.  Photo shows Pond A19, looking in through the levee breach at low tide.  All three “Island Ponds” are like this.  Sediment piled up to the level of Mean Higher High Water (MHHW) to create vast mud flats.  In this example, Ruddy Ducks paddle around in the borrow ditch channel while scattered shore birds forage on the mud banks.

Lower Coyote Creek – 10 Apr 2016, 10:22 AM.  And, at the same time, mud banks expose along the edges of Lower Coyote Creek.

Long-legged birds, like this Snowy Egret, often stand in high tide to catch fish that rush in to feed on bugs in the submerged mud flats.

You see this cycle every day, albeit a little more extreme in January and July:  mud banks and mud flats expose for hours at low tide, then submerge at high tide.  Planktonic fish and bugs drift considerable distances.  Bugs and clams planted in sediment either have a submergence/exposure survival strategy, or they don’t survive.  Birds and bigger fish time their arrival to coincide with tide changes, and bug movement.

Very short legged birds, like Least Sandpipers, arrive as the tide falls and mud flats are exposed.

Pond A6 – 2 Dec 2017, 3:56.  Tide is falling!  A very common marsh scene: Dabbling and diving ducks, that forage in deep water, take off as tide falls.  And, almost immediately, shore birds move in to pick over freshly exposed mud.

As tide fell on 2 December, Avocets came out to feed on exposed banks.  Many shorebirds in the tide zone follow the water line according to food availability and their own bill and leg-length characteristics.

Pond A19, 15 January 2017.  Rising tide.  It is not uncommon to see shore birds congregate to feed as tide rises in the ponds.  Both flooding and falling waters put mud dwelling insects on the move.  These birds know when to move in and grab the bugs.

Pond A19, 15 January 2017 – 30 minutes later.  Tiny birds left as water-level rose.  The table is now clear for deeper wading avocets for a few extra minutes.  I have thought about carefully documenting this tide and shorebird dance a number of times.  I haven’t gotten around to it.

6 Aug 2017, 3:00 PM.  Falling tide.  Extreme tides in Lower South Bay also induce extreme currents.  Above is a shot of the Railroad Bridge over Coyote Creek.  I took the photo after Pat Crain pointed out that we could actually see a drop in water elevation as it rushed away from us through the bridge struts.

Lower South Bay extreme tides provide a tremendous amount of physical mixing energy.  This impacts the water-sediment interface, and hence the biology of Lower South Bay.  From a wastewater treatment plant perspective, I look at this and think: “Wow!  What a great mixing motor for our denitrification tanks!”  (I may be alone in that view.)

People need to appreciate our extreme tides.  Tides have shaped, and will continue to shape, Lower South Bay geology.  Channels will scour.  Sediment will be resuspended. And, marsh height will rise as new sediment is deposited.  I am confident that extreme tide is our friend in Lower South Bay.

Know your Tides!  Tides in the Bay, and over most of the Pacific Coast, go in and out twice per day (two high tides and two lows).  The phenomenon is called “semi-diurnal tide.”  Our tides are also “mixed,” which means we experience one higher-high tide and one lower-low each day.

Screenshot of NOAA “Tides and Currents Pro” program.  The shot above shows a calculated prediction of tides on Monday, December 4th, for “Coyote Creek Tributary no. 1” which is located a short distance upstream from the confluence of Coyote Creek with Alviso Slough (Guadalupe River).  You can get the same current and predicted tide information at the NOAA website (just not as colorful display!)

On December 4th, the day after full Moon, higher-high tide occurred at 12:33 pm with a height of 9.6 feet NAVD.  Mean Higher High Water (MHHW) at this location is 8.4 feet NAVD and is indicated by a blue horizontal line.  Tides that exceed MHHW or go lower than Mean Lower Low Water (MLLW) are indicated as red zones on the chart.

The smaller horizontal strip in the upper middle summarizes 7-days of tide predictions from December 1st through the 7th.  In December, daily higher-high tides always happen during the day in SF Bay.  This is because the Earth is swinging towards perihelion in early January: gravity beats centrifugal force during this part of the year.  Lower-low tides occur at night.

In summer, this pattern reverses: higher-high tides come in at night; lower-low tides can be seen in the early part of the day.  Be warned though, you see this pattern only in places that experience “semi-diurnal mixed tides.”  Ocean basins that have diurnal tides or more basic “semi-diurnal” tides will not follow this winter-summer rule.

If you understand your local tide, you develop a sense for the movement of Earth and Moon relative to the Sun, which is pretty cool.

Predicting Sea Level Rise.  NOAA and other sites also show sea level trends from tide gauges in the U.S. and around the world. The NOAA website has a clickable map to drill down to your geographic area.  The screenshot below shows San Francisco Bay’s tide gauges (red and red/yellow pushpins) and location of meteorological or current data (yellow only and blue pushpins).  Clicking on any push pin pulls up a new screen and data summary for that station.

When you click on the San Francisco Golden Gate push pin, you see the pop-up on the left side in the above screen shot.

Click on “Sea Level Trends” at bottom of the pop-up menu, you will see the San Francisco tide gauge chart above – top left.  (For brevity, I made a four-panel collage of sea level rise rates for four gauges in or near SF Bay.)

I hear much excitement about the possibility that Sea Level Rise may drown marshes in Lower South Bay by 2100.  Currently, sedimentation rates in LSB marshes are on the order of a few inches to over a foot per year.  Sea Level Rise is measured at around 2 to 4 millimeters per year (8 to 16 inches per century), and the rate has not changed much over decades according to the gauges. I would say, the marshes are fairly safe at the moment.

You can view any tide gauge and sea level rise rates around the U.S. coasts.  Trust me, this can be quite addictive.  Here you can see that sea level has been falling slowly but steadily at the Crescent City gauge since it was installed after 1930.

“Falling sea” is a common trend along the North Pacific coast.  The trend becomes extreme at some gauges way north around Alaska.  It represents “post-glacial rebound (aka isostatic rebound).”  The sea is not falling; land in the far north is rising because it no longer bears the weight of ice-age glaciers. (To be honest, the gauge at Crescent City is probably more affected by plate tectonics and proximity to the Cascadia Subduction Zone, which is a slightly different “land level rise” issue.)

I focus on California tide gauges because we live here.  But, as you review gauges in other parts of the U.S. or the world, you will see that sea level rise rates are different in different areas.  The Gulf Coast and eastern seaboard are usually noted as experiencing greater rates as a result of land subsidence and continental movement.

Another good web resource is “Permanent Service for Mean Sea Level” (PSMSL).  This site shows tide gauge data for the entire world.  It is updated more frequently than the NOAA site, but trend charts are not quite as informative.

Sea level rise is such a hot button issue, that I would encourage everyone to become familiar with their local gauges.

Don’t fear the tide!  Adapt!





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