9.2-Foot Extreme Tide at Newark Slough


At 2:32 PM on October 9th, 2006, there was a 9.2-foot extreme high tide up Newark Slough. In the film clip below, you begin looking West along the Levee just South of Newark Slough in the Don Edwards San Francisco Bay National Wildlife Refuge. The camera is on the Levee, at what would normally be a pond at Sample Site 1 in the salt marsh. If you click on the triangle, you will see the tide flowing out of the Slough and into the salt marsh between Newark Slough and the Levee. Then you will pan around through the North, showing the flooded salt marsh, and end up looking East along the Levee.

The photograph below shows the pond at Sample Site 1 at a time between tides. The salt marsh is thickly overgrown with Pickle Weed and the pond is small, with a mud bottom.

Intertidal Sample Site 1 Pond

Compare view of the small pond to a photograph taken just at maximum tide on Monday, October 10th. It is a startling change.

Extreme tide, Sample Pond 1

The sample pond is only flooded like this at an extreme tide. Such extreme tides are infrequent. An ordinary high tide does not clear the banks of Newark Slough, so the salt marsh is not totally flooded and the water in the depression that creates the small pond continues to evaporate.

Both the cycle of tidal flow and the altitude of the intertidal zone are critical to the development of a salt marsh. If the altitude is between mean sea level and the mean high-tide level, the area is flooded at every tide, roughly twice a day, and it fails to develop into a salt marsh and remains a mud flat. This is sometimes called the “low marsh” zone.

If the area is at or above the mean-high tide mark, it has periods between extreme tides when it dries out, permitting the invasion of salt tolerant plants like Pickle Weed. Such areas develop into salt marsh, and are sometimes called the “middle marsh” zone.

Depressions within the “middle marsh” zone, however, retain water between tides and become small salt ponds. The salinity in such ponds varies widely, depending on evaporation between extreme tides. A tide sufficient to clear the higher land around the pond refreshes the pond, resetting the salt concentration to that of the Bay. Then the pond water evaporates, with salt concentration rising, until the next extreme tide.

Such salt ponds develop a unique micro-ecology, depending on the range of salt concentration. That range may change from year to year, as varying amounts of impounded fresh water are released into the Bay. Usually the Bay water salinity is near that of the ocean, about 34-ppt. This year, however, the heavy rain and snow fall in the mountains resulted in a high inflow of fresh water into the Bay. Apparently this release of impounded water continued well into the summer, and Bay salinity remained closer to 29-ppt or 30-ppt. The result was a substantial change in the micro-communities of such small salt marsh ponds.

The LaReviere Salt Marsh, East of the Visitor Center, is connected to Newark Slough. In between tides, and even during less extreme high tides, this salt marsh is not flooded. Shown below is a typical pond in the LaReviere Salt Marsh.

Pond in LaReviere Salt Marsh, not flooded.

Shown below is that same location during the 9.2-foot extreme tide on October 9th. The dark area, caused by mud on the pond bottom, outlines the location of the pond.

LaReviere Pond at high tide

Had I not carefully identified my sites by GPS location and by local reference points, it would be very difficult to find this pond during the extreme tide.

Even at an extreme tide, many areas along the edge of the LaReviere Salt Marsh are not flooded. These areas are slowly reverting to uplands. The “high marsh zone”, defined as the zone upwards of the mean high-water mark, is also called the “upland transition”. This is the area that slowly changes into the upland ecology around the Bay marsh lands.

Nothing in geology or ecology is fixed for long. As silt is deposited in the higher areas of the middle marsh, and as plants stabilize that silt, the middle marsh slowly evolves into the upland transition.

The salt marshes of the Don Edwards San Francisco Bay National Wildlife Refuge are undergoing rapid change away from the salterns of the salt production days. We are, literally, watching the evolution of salt marshes before our eyes. It is a unique opportunity to study the cause and course of this evolution.

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