On October 11th, 2015, 18 volunteer bike and mapping aficionados gathered at my place to work on the Cistern Mapping Project — an endeavor to physically document the 170 (or so) Cisterns in San Francisco. There exists no comprehensive map of these unique underground vessels. The resulting map is here.

I personally became fascinated by them, when working on my Water Works project*, which mapped the water infrastructure of San Francisco.

The history of the cisterns is unique, and notably incomplete.

The cisterns are part of the AWSS (Auxiliary Water Supply System) of San Francisco, a water system that exists entirely for emergency use and is separate from the potable drinking water supply and the sewer system.

In the 1850s, after a series of Great Fires in San Francisco tore through the city, about 23 cisterns were built. These smaller cisterns were all in the city proper, at that time between Telegraph Hill and Rincon Hill. They weren’t connected to any other pipes and the fire department intended to use them in case the water mains were broken, as a backup water supply.

They languished for decades. Many people thought they should be removed, especially after incidents like the 1868 Cistern Gas Explosion.

However, after the 1906 Earthquake, fires once again decimated the city. Many water mains broke and the neglected cisterns helped save portions of the city. Afterward, the city passed a $5,200,000 bond and begin building the AWSS in 1908. This included the construction of many new cisterns and the rehabilitation of other, neglected ones. Most of the new cisterns could hold 75,000 gallons of water. The largest one is underneath the Civic Center and has a capacity of 243,000 gallons.

The original ones, presumably rebuilt, hold much less, anywhere from 15,000 to 50,000 gallons.

Armed with a series of intersections of potential Cistern Locations, the plan was to bike to each intersection and get the exact latitude and longitude and a photograph of each of the cistern markers — either the circular bricks or the manholes themselves.

We had 18 volunteers, which is a huge turnout for a beautiful Sunday morning. I provided coffee and bagels and soon folks from my different communities of the bike team, Exploratorium and other friends were chatting with one another.

One way to thank my lovely volunteers was to provide gifts. What I made for everyone were a series of moleskine notebooks with vinyl stickers of the cisterns and bikes. I was originally planning to laser-etch them, but found out that they were on the “forbidden materials” list at the Creative Workshops at Autodesk Pier 9, where I made them. Luckily, I always have a Plan B and so I made vinyl stickers instead.

Here I am, in desperate need of a haircut, greeting everyone and explaining the process. I grouped the cisterns into blocks of about 10-20 into 10 different sets. This covered most of them and then we paired off riders in groups of 2 to try to map out the best way to figure out their ride.

Some of the riders were friends beforehand and others became friends during the course of riding together. Here, you can see two riders figuring out the ideal route for their morning. Some folks were smart and brought paper maps, too!

Here are the bike-mappers just before embarking on their day-of-mapping. Great smiles all around!

I would have preferred to ride, but instead was busy arranging the spreadsheet and verifying locations. Ah, admin work.

How did we do this? Simple: each team used a GPS app and emailed me the coordinates of the cistern marker, along with a photo of the cistern: the bricks, manhole or fire hydrant. I would coordinate via email and confirm that I got the right info and slowly fill out the spreadsheet. It was a busy few hours.

The hills were steep, but fortunately we had a secret weapon: some riders from the Superpro Racing team! Here is Chris Ryan crushing the hills in Pacific Heights.

One reason that we traveled in pairs is that documentation can be dangerous. Sometimes we had to to put folks on the edge or actually in the street so they could get some great documentation.

So, how many cisterns did we map? The end result was 127 cisterns, which is about 75% of them, all in one day. We missed a few and then there were a series of outliers such as ones in Glen Park, Outer Sunset and Bayview that we didn’t quite make.

And the resulting map is here. There are still some glitches, but what I like about it is that you can now see the different intersections for each cistern. These have not been documented before, so it’s exciting to see most of them on the map.

What did we discover?

Most of the cisterns are not actually marked with brick circles and just have a manhole that says “Cisterns” or even just “AWSS” on it.

What I really enjoyed, especially being in the “backroom” was how the cyclists captured the beautiful parts of the city in the background of the photos, such as the cable car tracks.

Also, the green-capped fire hydrants usually are nearby. These are the ones that get used to fill up the cisterns by the SF Fire Department.

A few were almost like their own art installations, with beautiful brickwork.

The ones in the Sunset and Richmond district are newer and are actually marked by brick rectangles

Thanks to the AMAZING volunteers on this day.

* Water Works is supported by a Creative Code Fellowship through Stamen Design, Autodesk and Gray Area.