Oakland Geology

The Sausal Creek watershed is full of anomalies and questions from its headwaters to its San Francisco Bay outlet. Here I’ll look at the top end of Sausal Creek — the part that isn’t Sausal Creek. The creek originates where three different tributaries join: Shephard Creek, Cobbledick Creek and Palo Seco Creek.

The divide between Shephard and Cobbledick runs up Chelton Drive, then Darnby and Carisbrook Drives, up to Skyline. The divide between Cobbledick and Palo Seco runs mostly up Castle Drive to Skyline, so if you know the area these are easy to visualize.

That map, from the Alameda County Flood Control and Water Conservation District, has Sausal Creek proper beginning at the junction of Shephard and Cobbledick, which is also where the Hayward fault crosses the creek (more about that below). But because that’s in a culvert deep beneath the Warren Freeway, for purposes of this post I prefer to put the origin a little farther down, in an easy-to-miss opening east of the parking lot of the Montclair Golf Course driving range at the head of Dimond Canyon. That’s where Palo Seco Creek, the third tributary, comes in from the redwood-filled canyon of Joaquin Miller Park.

So with that settled, let’s look at the headwaters on the geologic map. This area includes a wide variety of rock units — the Sausal Creek watershed touches more different rock types than any other Oakland stream.

Don’t worry, I won’t go into the rocks, although there are a lot of them and they’re interesting . . .

I’ve added the Hayward fault to the map, as a thick red line, just to show how different the rocks upstream and downstream are. That’s because motion on the fault has been dragging the west side to the north for a few million years. That explains two major peculiarities of Sausal Creek, the first being its lumpy longitudinal profile.

I made this stream profile by walking down the creek from the top of Eastwood Court to the Bay, recording elevations with my smartphone altimeter.

A normal stream profile describes a nearly smooth listric curve — steep at the top and level at the bottom. The bottom of the curve represents what’s called the base level for the whole stream, sea level in this case. A stream with a nearly perfect curve is said to be at grade. Two basic things will put kinks in that curve: rocks that are especially hard or soft, and changes in base level. For instance, ice ages lower the sea level by hundreds of feet, and streams have to adjust during that time by digging down their beds toward the new base level. (I alluded to this in my last post with respect to Lake Merritt.)

There’s a big discontinuity in this curve right where the Hayward fault crosses, just above the 3 kilometer mark. The lower half, Sausal Creek, is at grade, even though it crosses hard sandstone in Dimond Canyon and young sediment farther down. But the fault has ripped its head off and put on another head — the Shephard-Cobbledick-Palo Seco system. It’s a Frankenstein creek. I think this head transplant has happened more than once.

Sausal Creek, over the last million years or so, has done fine even with its head ripped off and replaced. The worst that might have happened is that it had less water in it for a while. But in the upper creek system, these changes have drastically affected its base level.

Picture it: the two sides of the Hayward fault are moving past each other at about 10 millimeters a year, or a kilometer every 100,000 years. So for a good long time, Shephard Creek flowed down against the high rocky ridge of the Piedmont hills. Very likely it turned north for a long ways, the way Temescal Creek does today, before flowing around the north end of the ridge. As its route to the Bay got longer and longer, the slope of the stream grew gentler as it remained at grade. The effective base level, that is to say, was up around the elevation of the Thornhill district.

Then along came Dimond Canyon, moseying up on the far side of the fault, and at some point Shephard Creek switched over to that route. All of a sudden, it had a lower base level. It was not at grade. So it started eroding downward into its streambed and eroding uphill, trying to re-establish that nice listric profile.

When that happens to streams, what geologists call a knickpoint appears in the profile. The extreme case of a knickpoint is a waterfall, but more often they’re just rapids. In Shephard Creek, there appear to be two knickpoints.

Pay attention next time you ride down Shepherd Canyon Road. There’s a nearly level stretch in the road between Saroni and Escher Drives, where the railroad trail meets the road, then a steep “rapids” below. The other knickpoint is under the landfill of Shepherd Canyon Park, where the creek is buried in a culvert. A more carefully made profile would show it better, but that may not be possible.

I could conjecture a story that accounts for these features, but there’s a lot I don’t know so it would just be armwaving. For instance, the stream profile is based on the elevations of the roadway rather than the actual streambed except for the part between Shepherd Canyon Park and Mountain Boulevard (which is so thickly wooded I don’t recommend you visit, even though I did) and the one data point at the golf club. For another, the history of vertical movements along the Hayward fault is almost completely unknown, other than that the hills on the east side are rising today at about a millimeter per year. So enough about that.

I mentioned that the creek has two major peculiarities, and here’s the second one. The upper part of the Sausal Creek watershed is not a pretty, textbook stream network shaped like a nice tree — what geologists call a dendritic drainage pattern. It’s more like a bush in a gale, and all of the streams that cross the fault are warped. Here’s how it looks in the set of stream maps on the Oakland Museum website:

And for comparison here’s Temescal Creek:

And Arroyo Viejo, the weirdest of all.

There’s a struggle going on between the stream’s innate tendency, driven by gravity, to dig itself a home with an optimal shape and the motion of the ground beneath, driven by tectonics, that keeps messing it up. The shapes of the streams, and the landscape they live in, are a snapshot of that struggle. They remind me of the shapes of trees high on windy mountains — although one case involves organisms and the other purely physical systems, the similarities are tantalizing.

Geologists are starting to explore this topic with computer models. A recent paper in Geophysical Research Letters, with the dry title “The Role of Near‐Fault Relief Elements in Creating and Maintaining a Strike‐Slip Landscape,” has some state-of-the-art animations that address the exact situation of Oakland’s fault-crossing streams. You don’t need to understand all the modeling details — I certainly don’t — to enjoy the illustrations and the movies.

This entry was posted on 30 September 2019 at 8:01 am and is filed under Hayward fault, Oakland streams and water, Sausal Creek watershed. 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.