Archive for the ‘Sausal Creek watershed’ Category

Anomalies of Sausal Creek: Dimond Canyon

14 October 2019

This is the second of four posts about the peculiarities of Sausal Creek, going from its headwaters to the Bay. Here I’ll address Dimond Canyon, the 2-kilometer segment between the Warren Freeway and the flats of Dimond Park. The steep walls of the canyon, which is several hundred feet deep, are entirely hard sandstone of the Franciscan Complex, part of the Piedmont block.

This is the same stone quarried for decades in Rockridge (the Bilger quarry) and the land that would later become Piedmont (the Blair quarries and the Davie Stadium quarry). In fact the Diamond Cañon Quarry was one of two here in the canyon. It’s now occupied by the Zion Lutheran Church, as seen here from across the canyon.

The quarry scar appears on this terrain map as a big round nick in the canyon wall next to Park Boulevard.

A while ago in this space I described Dimond Canyon as a classic water gap — a stream-cut gorge crossing a bedrock ridge that otherwise seems impenetrable.

Geology textbooks will tell you there are two ways for streams to make a water gap. In the first way, the stream was there first (an antecedent stream) and a ridge of resistant rocks rose up around it. In dynamic California, this is a straightforward explanation of our water gaps. In the second, the ridge was there first, inherent in ancient deformed rocks buried under younger strata, and the stream (a superposed stream) cut down to, then into it while stripping off the overlying material. That’s how they explain the Delaware Water Gap and other examples in the gentle Appalachians.

Dimond Canyon is actually a semi-classic water gap. Yes, the ridge it crosses must have risen while the stream was cutting down, but the story is complicated by the fact that the watershed upstream lies across the Hayward fault, and is constantly being moved to the right. This means the canyon has hosted streams from several different watersheds over the past million years or so.

Therefore the streams feeding Sausal Creek today could not have dug the canyon; some predecessor watershed did it. There must have been gaps and surges in the water (and sediments) flowing through this canyon. If we ran things backward a million years, what would it show? The exercise would be blurred by serious uncertainties, but the matter is not beyond all conjecture.

I beg your indulgence as I present some slides from my talk to the Friends of Sausal Creek last month. They’re Google Earth views looking west across the fault. Here’s today, with the fault trace shown in red.

The view may be a bit confusing as I rewind the motion on the fault at about 10 millimeters per year. The far side looks the same because we’re focusing on it while it moves leftward, toward San Leandro. For a long time, Sausal Creek has been carried past small watersheds that, like today’s, could not possibly have carved Dimond Canyon. But about a million years ago, Dimond Canyon would have lined up with the watershed of Arroyo Viejo.

This looks promising because the watershed (the part above the fault) is about twice the size of Sausal Creek’s, giving it roughly twice as much water and cutting power to match.

But to make the canyon, you have to have something pushing up the ridge while the stream across it keeps cutting its way down. There’s nothing obvious that would have been pushing up the bedrock ridge at this time.

Going back a bit further, though, we line up with the great big watershed of San Leandro Creek, a dozen times larger. This stream has plenty of cutting power, evident in the canyon it’s dug where the dam and reservoir sit.

And finally, we have a mechanism here for uplifting the ridge that Dimond Canyon cuts across. The hills of San Leandro consist of a large slab of gabbro so big and strong that it deflects the Hayward fault slightly. Back when the sandstone of Dimond Canyon was grinding past the gabbro of San Leandro, the jostling between these two bodies of rock, caught in a vice by the geometry of the fault (a restraining bend), would have pushed both sides upward because that’s the only way out of the vice. And all the while San Leandro Creek would have been cutting a nice deep water gap as that hard rock rose.

Eventually, inevitably, the fault carried the water gap out of reach, and ever since then Dimond Canyon has housed lesser creeks for episodes of a few hundred thousand years. Sausal Creek trickles down the canyon today not doing much to it, the shrunken tenant of a structure built by a mightier maker.

This story (and that’s all it is really) appeals to me because it would also explain the presence of the Fan — the swath of gold on the geologic map representing Pleistocene sediment.

I’ve always regarded it as a fossil alluvial fan because of its shape on the map, but maybe that’s accidental. Maybe it’s just a chunk of old East Bay land that was lifted along with the Piedmont block, or washed off of it afterward.

I first posted about the problem of Dimond Canyon more than 10 years ago. Takes a while to figure out some things.

Anomalies of Sausal Creek: The Headwaters

30 September 2019

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.

A circumambulation of Shepherd Canyon

18 February 2019

I seem to give myself odd assignments. The latest one was to take a hike around the crest of Shephard Creek’s watershed, better known as Shepherd Canyon. Only during the final mile of that 6-mile walk did I realize what it meant: an outing exclusively on ridge roads.

I got the idea from my circumambulation of Claremont Canyon three years ago. That walk features a variety of rock types and a good set of views into Oakland’s best wineglass canyon — a stream valley with a wide upper watershed and a narrow outlet where it crosses an active fault. Most of Oakland’s major streams that cross the Hayward fault have wineglass valleys that are more or less well-formed (and faceted spurs between them), but you may have to scrutinize them pretty hard to see them, let alone walk their rims.

Shepherd Canyon is another of our wineglass canyons where the roads make this exercise feasible — and an exercise it is, with a thousand-foot elevation gain. The trouble is, you don’t get clear views into the canyon itself. Whereas Claremont Canyon is a viewshed as well as a watershed, Shepherd Canyon’s watershed has an inside-out viewshed. Thanks to our high parklands, this ridgetop walk affords good views around the canyon’s outer side.

Enough preamble — let’s amble. The route starts and ends in Montclair on the geologically friendly 33 bus line. Here’s the layout.

Those little red blips are mile markers. I took this counterclockwise because I prefer steep uphills to steep downhills, and the south end is steep. A bicyclist might prefer the other way, but parts of this route are footpaths.

You could start and end this walk at the end of the 33 line in front of the Safeway, but I started from the Snake Road stop and finished at the LaSalle Avenue stop. Most of the first mile is a bit of a chore along busy Mountain Boulevard, but these days the rain has made the creek noisy, and you’ll hear it down in the woods as you cross the lowest point near the Park Avenue junction.

Be sure and get off the road on this nice path.

It goes around the flattened hilltop housing the Joaquin Miller Elementary School and Montera Middle School.

There once was a rocky hill here, occupied by a Scout camp (which is how Scout Road got its name), but it was vacated and leveled as a handy source of stone and used (I believe) as fill for the Warren Freeway nearby. In any case, the path takes you to the foot of Shepherd Canyon’s perimeter ridge, where you’ll turn right up Mountaingate Way. This view past the foot of Mountaingate looks into the valley of Cobbledick Creek, a tributary of Shephard Creek that runs down along Scout Road. In the next mile you’ll climb 800 feet, pretty steadily.

Soon enough you join Castle Drive and start seeing rocks. Now it’s time for the bedrock map.

The ridge is made up of serpentine rock (sp on the map) plus its typical blocks of blueschist. One of the city’s best blueschist boulders sits where Castle Drive starts, and some of the homes along Castle use the blueschist in their landscaping.

The serpentinite is a sloppier, greener stone that you’ll see along the roadside. Let me warn you that hiking on Castle is a challenge — the road is narrow, there’s no sidewalk, and nobody expects pedestrians. Keep your ears open and step off the road as cars approach.

Where Castle starts to turn north, veer off the road and take the little-traveled West Trail, part of Joaquin Miller Park. You’ll thank me, as I said when I took you down it a few months ago. It’s here that your first views open up.

And the rocks change as you start on the trail. From here on out you’ll be mostly on sandstone. Just for the record, you’ll pass through the Joaquin Miller Formation (Kjm), Oakland Conglomerate (not labeled), Shephard Creek Formation (Ksc), Redwood Canyon Formation (Kr), unnamed Eocene mudstone (Tes), Sobrante Formation mudstone (Tsm) and just into the edge of the Claremont Shale before descending through the same units on the way down.

Castle Road ends at Skyline Boulevard. After a few steps on Skyline, duck into the woods and take the Scout Trail parallel to Skyline, where you’ll meet these guys at the top of the ridge — Oakland Conglomerate.

Where the trail ends at Moon Gate, take Skyline for a few hundred yards, passing the Waterloo Staging Area (unless you’re up for a stroll on the West Ridge Trail, which rejoins this route at Skyline Gate), and take Wilton Drive up onto the ridgetop. It skirts the edge of Redwood Regional Park and has one of Oakland’s best views.

Move on and take Burton Drive, then Shirley Drive. At the end of Shirley is an unmapped trail through Redwood Park that starts with this bench, nestled among boulders of Redwood Canyon Formation sandstone.

Not quite the halfway mark, but just the place to take a rest and a good look around.

The trail descends to the Skyline Gate Staging Area. From there, take Skyline to its confusing junction with Shepherd Canyon/Pinehurst Roads. Don’t take any of those roads — go up Manzanita Drive instead, along the ridgetop. Where the power line crosses the ridge is a fine exposure of the Sobrante Formation’s shale.

This is an unusually good exposure of the Sobrante, which is very sloppy stuff along Skyline, because up here it’s grading into the harder chert of the Claremont Shale.

The two units are stratigraphically continuous, with an arbitrary boundary between them. You’ll see a lot of this rock along the road as you make your way north, then down to the junction of Skyline and Snake Road.

At this spot you overlook Shepherd Canyon on the left, Thornhill Canyon on the right, and the mighty Bay in front as you start down. Do stop at the fire station here and top up your water supply.

The descent along the lip of Shepherd Canyon is straightforward: Take Colton Boulevard to Asilomar Drive to Drake Drive. Again, pedestrians are not expected, but the roads are generally wide enough to accommodate everyone without jostling or stir. Here and there you can see into the canyon. Watch its walls grow close as you come near the narrow part of the wineglass.

When Drake meets Magellan (I know, those two explorers never actually met), the ridge road is finished. It only remains to take the unobtrusive path leading through the trees from 2133 Magellan down to the Railroad Trail — when else will you ever have the chance? — and on into Montclair to catch the bus.

The nice thing about this time of year is that many of the trees are leafless, so you can see more of the surroundings.

That and the weather is cool. I always seem to assign myself weird walks in the high hills during summer, which can be brutal. Better to get out there now.

The Dimond Canyon water gap

15 May 2017

In a city full of geologic features, Dimond Canyon stands out as a classic example of a water gap. But it can be hard to see, even from the prime viewpoint of Leimert Bridge.

Let’s abstract ourselves by studying the overhead views shown in maps. Google Maps, with the terrain view turned on, is where I like to start.

Compare Dimond Canyon, cutting straight through the bedrock ridge of the Piedmont block, with Indian Gulch (Trestle Glen) on the left, a conventional stream-cut valley that fans out against the ridge.

For a starker view of the topography, I like to consult old USGS maps like the 1897 Concord quadrangle, made before most of the area was built up and dug into.

Here we can see that the ridge reaches the same elevation on either side of the canyon — without the canyon cutting through it, this would be a continuous crestline.

There is no sign, either, that Dimond (“Diamond”) Creek cut its way through by headward erosion. That would have left tributaries fingering off on either side, like those visible in the contours above Indian Gulch. Indeed, the single little tributary in the canyon is actually a hanging valley that has to descend steeply as it meets Dimond Creek — not as spectacular as those in Yosemite Valley, but with the same basic configuration.

Finally, we can look at the bedrock evidence in the geologic map.

The whole area around the canyon is mapped as Franciscan sandstone (Kfn), with no hint of faulting or other structure that might have favored the formation of a canyon here. Consider the well-developed valleys above the canyon, guided into existence by the rock-crushing Hayward fault, or the more subtle topographic features where the southern edge of the Franciscan bedrock meets old alluvium.

What we have here, then, is a genuine water gap — a deep pass in a mountain ridge with a stream flowing through it. Geology textbooks will tell you there are two ways to make one. One is for a river to uncover an ancient ridge as it strips the countryside of its sediment cover. The classic case is the Delaware Water Gap in Pennsylvania. The other is for a river to sit there, doing its thing, as the land rises up around it. The Central Valley has good examples at the foot of Del Puerto Canyon . . .

. . . and the Berryessa water gap west of Winters.

What’s odd about the Dimond Canyon water gap is that it’s being carried along the Hayward fault. Every few hundred thousand years, then, it gets itself a new headwater catchment. Today its catchment is Shephard and Palo Seco Creeks. Once upon a time, though, it must have carried the waters of San Leandro Creek. Coming up: Temescal Creek.