Which county is that gull in?

Various folks, including me, were speculating on the exact location of the Marin – San Francisco County Line while enjoying the Iceland Gull.  I knew that all of the water was in SF, but didn’t know exactly what else was in San Francisco.  Well, I haven’t found a good written description of the county line online and I can’t figure out how to show county lines on Google Maps, so I reverted to our good old friend, the USGS.

If you check the map below, you’ll see that all of the water in Horseshoe Bay is indeed in SF.  Additionally, all of the Coast Guard Pier and the rock jetty coming out from the western edge of Horseshoe Bay is also in SF.  If you literally take the county line as drawn (shown as a solid black line that is intermittently dashed), then the bird today was entirely in San Francisco while I observed it.

I don’t recall where Steve originally found the bird nor whether other folks have observed the bird northeast of Point Cavallo, which of course would be in Marin.

2/10/2012 Update: Speaking of how the county lines were originally drawn, Both Dan Singer and Dave Quady shared the actual definition of the county lines:

23138.  The boundaries of San Francisco city and county are as follows:   Beginning at the southwest corner, being the northwest corner of San Mateo, in the Pacific Ocean, on the extension of northern line of T. 3 S., of Mount Diablo base; thence northerly along the Pacific Coast, to its point of intersection with the westerly extension of the low-water line on the northern side of the entrance to San Francisco Bay, being the southwest corner of Marin and northwest corner of San Francisco; thence easterly, through Point Bonita and Point Cavallo, to the most southeastern point of Angel Island, all on the line of Marin; thence northerly, along the easterly line of Marin, to the northwest point of Golden Rock (also known as Red Rock), being a common corner of Marin, Contra Costa, and San Francisco; thence due southeast four and one-half statute miles to a point established as the corner common to Contra Costa, Alameda, and San Francisco; thence southeasterly, on the western line of Alameda County to a point on the north line of T. 3 S., R. 4 W., M D. B. & M.; thence westerly on the township lines and an extension thereof to the place of beginning. The islands known as the Farralones (Farallons) are a part of said city and county.

And here’s a definition of low-water mark:

The level reached by seawater at low tide or by other stretches of water at their lowest level.

I’ve also done a little bit more research and found a historical map of Ft. Baker from 1925:

One can clearly see that neither the Coast Guard Pier (and beach where most of the interesting gulls were being seen) did not exist in 1925 and thus were certainly not present at the time the county lines were drawn.

And finally, here’s a comprehensive discussion of all changes to the county lines since CA was created in the first place, which among other things, shows that Marin and San Francisco passed Angel Island back and forth a few times and when Alameda Naval Air Station finally grew into SF:


To summarize from a birder’s perspective:

  • All of the water below the low-water mark is in San Francisco
  • All of the land above this line is in Marin
  • EXCEPT, all man-made structures or additions, including the Coast Guard Pier and beach, are in San Francisco

To be sure, figuring out where the low-water mark can be challenging anywhere well away from low tide, but only for gulls very near the shore, and not for birds which were at the edge of the water on the beach of the Coast Guard pier.