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Weep Revelation

Cris

For a couple of years now Wayne and I have been puzzled about the driving forces for the environmental, and hence microbiological, circumstances of a site just north of Alviso. It is a trough about 50 meters long that we have named ‘the weep’ for it is always moist to some degree. If you search the Hidden Ecologies WWW site for ‘weep’ you will find many mentions of this modest, yet entertaining, depression that runs along the side of the Southern Pacific Railroad grade. Over the months we have puzzled about the role of tidal waters and the adjacent salt ponds in driving somewhat counterintuitive and complex cycles of wet and dry at the site. In any event the weep has become a special place to watch and ponder.

Drawbridge to Alviso - 1928

Figure 1. A mosaic of aerial images c. 1928 showing the railroad corridor from Drawbridge to Alviso. These photographs have recently appeared on the South Bay Restoration Project WWW site by way of the San Francisco Estuary Institute who in turn references the Cargill Corporation. They are fantastic, thanks to all.
http://maps.southbayrestoration.org/sbsp/viewer.htm

This morning I unraveled yet another reason the weep is special. While visiting the South Bay Salt Pond Restoration Project WWW site I noticed that the interactive project map developed by the fine folks at the San Francisco Estuary Institute (SFEI) now offers a layer featuring aerial survey photographs from 1928. I then spent a pleasant hour or so wandering through these new (old) photographs and found many features of interest. Up north the photographs show several of the smaller salt operations and landings that existed before consolidation and flood control projects. The community of Drawbridge is shown, fat and sassy before the grim advent of Salt Ponds A20 and A21 on Station Island. And the photographs show the stretch of railroad running from Drawbridge to Alviso too. The railroad grade was constructed in 1880 and it is this bank against which our weep site lies.

Since the aerial photographs provide a glimpse of the weep site before Salt Ponds A13 and A15 were constructed I took a good close look. My curiosity was quickly aroused regarding features in the 1928 aerial photographs so I then referred back to the elegant 1858 US Coastal Survey T-sheets hosted by SFEI to take a look at the marsh before the railroad was built. And there I found a large marsh channel immediately south of Coyote Creek in the general vicinity of the weep.

Map layer comparison

Figure 2. This small snippet from the Photoshop montage shows common features on all three layers. The scene is a 325-meter square near the weep site.

There then ensued a Photoshop session during which I merrily assembled and aligned layers containing survey data from the 1858 T-Sheet, the 1928 aerial photographs, and a contemporary 2007 view from Google Earth. For the purposes of this informal exercise the three layers aligned nicely and it was possible to find features that appeared on all three layers such as the minor marsh channel running from mid-bottom upward in Figure 2. It is always a treat to realize that a small graphic squiggle on the T-Sheet you are studying is not an stylistic flourish but rather a reasonably accurate encoding of a minor feature in a vast marsh.

In my Photoshop sandwich of years I located the weep location on the contemporary view and marked it with a red dot. It was then an easy matter to examine the location of the red dot on the other image layers. As shown in figure 3, on the 1858 T-Sheet the weep coincides exactly with the northern edge of the marsh channel that caught my attention in the 1928 aerial photographs.

The weep vivinity in 1858

Figure 3. Vicinity of the weep cropped from T-Sheet 676. The weep’s location occurs on the northern, and deeper, edge of Gray Goose Slough, a marsh channel that does not exist today. http://maps.sfei.org/tsheets/viewer.htm

On close inspection, the remnants of this marsh channel are clearly visible in the bottom of Salt Pond A16 (the pond to the east of the railroad grade) as shown in Google Earth. Referencing the Oakland Museum of California’s Baylands & Creeks of South San Francisco Bay I was able to attach a name to our marsh channel – the Gray Goose Slough.

The weep vivinity in 2007

Figure 4. Vicinity of the weep cropped from a montage of 2007 Google Earth images.
Google Earth, 37.4483°N. 121.976°W.

Having determined that the weep site was once a major marsh channel it was fun to return to the 1928 aerial photograph that started this train of thought and to reflect on the nature of the site. By 1928 the railroad grade had been in place for almost 50 years but the salt evaporation ponds now flanking the weep were yet to be built. The 1928 aerial photographs reveal a vast marsh. The southern edge of Drawbridge (top of Figure 5) is densely occupied with piers extending into Coyote Creek. The railroad grade follows its straight line southward across the marsh, passing over a small bridge immediately north of the weep site. At the weep site itself there appears to be no bridge, suggesting that Gray Goose Slough is dammed, at least in part, by the railroad grade. For the railroad to cross this channel without a bridge there must have been a substantial amount of fill underneath the tracks as they crossed the marsh channel.

One wonders whether there were culverts to allow water passage below and through the railroad grade or whether the railroad formed, in effect, a complete dam at that location. If the dam scenario is correct then both sides would still be connected to tidal flow but in ways quite different to the previous natural state. In the 1928 photograph the Gray Goose Slough channel seems broad and robust on the Bay side (west) of the railroad and attenuated on the other side suggesting to me that the railroad has caused some form of hydrological change

The weep vivinity in 1924

Figure 5. Vicinity of the weep cropped from my original starting point – the 1928 aerial photographs from SFEI.
http://maps.southbayrestoration.org/sbsp/viewer.htm

The weep has always seemed a special place as we walk out the levee from Alviso. Even a casual visual assessment of this stretch of ditch finds it to have a different and varied character. In a Betsy Dyer-esque fashion, the exuberant and spatially varied colors of the weep surface suggest there is something more complex at play. There is no shortage of possible drivers for complexity. The adjacent salt ponds, mechanical vibration by the railroad, tidal action in the sometimes tidal ‘stream’ east of the railroad, varied schedules for filling Salt Pond A15, and our wet/dry season patterns must all contribute to the nature of this place.

Discussing the weep

Figure 6. Wayne Lanier and companion standing at the weep.

We now have another factor to consider in the fact that standing at the weep places us above what was once a substantial marsh channel. For instance, I wonder if the former marsh channel serves as a conduit for seepage from adjacent Salt Pond A15 to the weep? What was the source of the fill that now makes the weep surface and does it differ substantially from the natural ground? Does this fill shape the character of the weep?

One Response to “Weep Revelation”

  1. Dave McNabb Says:

    That is fascinating stuff. It is cool that you were able to gather all those old photos and piece the puzzle of the weep together like you did. It is also cool that people took the time back in the day to take these photos and document the area so that in present day we can see how things have changed. I think you solved the mystery of the weep.