Oakland Geology

Once upon a time, we used to produce a lot of our water from local wells, but for the last century we’ve retired them as the aquifers were drawn down or polluted. So I’m always surprised and intrigued to see wells still at work. This is on Willams Street in San Leandro.

The location is a big educational complex comprising John Muir Middle School, San Leandro Adult School, and Woodrow Wilson Elementary. Presumably the well is for watering the grounds, not supplying the drinking fountains.

The water is a remnant of the once-productive San Leandro Cone, a body of sediment full of groundwater supplied by San Leandro Creek. The geologic map shows it as a set of fingers radiating from Lake Chabot, labeled “Qhl.”

Qhl stands for “Quaternary Holocene levees.” The Quaternary Period includes the last 2.6 million years of Earth history, and the Holocene Epoch is the final portion of the Quaternary, the time since the glaciers last melted. I think of the Holocene as the geological present.

Levees form when floods regularly spill over a riverbank — the moment rushing floodwater leaves the river, it slows down and immediately drops most of its sediment load. Repeat this enough times and a low rise builds up on both sides of the river: a pair of levees.

The map explanation says about Qhl, “these deposits are porous and permeable and provide conduits for transport of ground water. . . . Abandoned levee systems have also been mapped.” And that explains the well water of Williams Street — the street runs right down the middle of a former course of San Leandro Creek. Here’s a closeup.

The map shows that the Qhl unit coincides with an extremely gentle rise in the ground about 10 feet high and a quarter-mile across. Natural levees are rare to see, because humans build them up higher for “flood control,” but you’ll easily see the rise if you walk away from Williams Street a couple blocks and look back at it.

So San Leandro Creek once ran this way, must be thousands of years ago. Something perturbed it enough to cut a new streambed in another direction, and this abandoned course filled in. But it still carries water underground.

The northernmost finger of Qhl in the San Leandro Cone leads to the former site of Fitchburg, where a major wellfield once supported East Oakland with groundwater.

This entry was posted on 21 August 2017 at 8:02 am and is filed under Oakland streams and water. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.