Resolving the County Line?
For the last couple of days, I have been on and off the phone with the Contra Costa County Assessor's Office trying to clear up this county line issue. But first, thanks to Aaron (I guess? sure did open a can of worms) for calling my bluff on my original post. I looked at the online Contra Costa GIS map, saw the line was straight and cut through the riparian corridor, and thought "oh, the Caltrans map is wrong, therefore birders have been right all along."
Anyway, the conclusion is that the county line shown on the Contra Costa and Alameda GIS parcel maps is the correct, definitive county line. The Caltrans Map and USGS topo maps are close, but incorrect. And yes, the county line does not follow the path of the creek. The county line was drawn in the 1800s as the boundary between two Spanish landgrants: Rancho San Antiono and Rancho San Pablo. The line was drawn to bisect El Cerrito Creek, but where Creekside Park currently exists, the creek used to have a large estuary. So the line was drawn down the middle of the creek to where it reached its estuary (at about Carlson Blvd.) and then the line was drawn through the estuary from where the creek entered the estuary to a large shellmound on the estuary's Western edge. Years later, the estuary was destroyed and the creek channelized, but the county line remained unchanged. And the creek's manmade channel was not built to follow the county line. Although it is close. Today with the creek developed on both sides, the destruction of the estuary and the channelizing of the creek, it leaves some oddities like buildings in Albany that have some of their parking lot in El Cerrito and vice versa but that is the official county line as used by both counties!
The assessor’s office informed me that the park was never officially surveyed so the line shown online could be slightly off, but without hiring an official land surveyor there would be no way to find the exact line on the ground. I was also told that if the line shown on the County maintained GIS maps was wrong, it would likely be off by a very small amount, at very most misplotted by a foot or two, but likely not even by this much. And that being said, Alameda and Contra Costa County GIS Parcel maps approximate the county line in the exact same place, so that is probably extremely close to or is the real county line, and is definitely the absolute best we can do as birders.
So to approximate the county line in the park: West of the Eastern edge of the basketball courts the entire park is in Contra Costa. From the dead end to Carlson Blvd. (so where most have seen the Tennessee) the county line runs pretty much right down the middle of the creek. Between the dead end and the basketball courts, the county line falls between the middle of the creek and the top of the creek’s southern bank. So to see a bird in Alameda in this section, it seems you would have to see it over the earthen bank on the creek’s South side, actually out from above the water.
This means a couple things: Most easily, how people have been deciding which county the Tennessee is in holds true, as the Tennessee has rarely been seen West of the dead end, so the creek is in fact split down the middle. Secondly, where I saw the Mourning Warbler in 2019 was well West of where the black fence starts on the East edge of the basketball courts, putting it entirely in Contra Costa. It might be a tough call to say that bird was ever in Alameda, but I know it did move a bit.
Anyway, best of September,