Oakland Geology

This is the third of four posts covering the Sausal Creek watershed from hill to Bay. It features the part of the creek between Dimond Canyon and Foothill Boulevard. The early Anglo landowners were quick to establish fruit orchards and berry patches there, giving rise to a settlement and district out east in Brooklyn Township they called Fruit Vale.

That was just the right name for this distinctive locality. Its alluvial land, flat, tractable and naturally well watered, was ideal for planting. I say “planting” rather than farming because within a few decades — the commercial lifespan of an orchard — the farms gave way to grand homes with lush landscaping. That’s the “Fruit” part. The “Vale” part refers, as the word has since the 14th century, to a wide, flat lowland between gentle hills with a stream in it. An 1888 pamphlet described it as “one vast garden,” its roadway “lined with elegant residences and villas surrounded by beautiful lawns and gardens. . . . The soil is easily cultivated and everything planted in it thrives.”

The creek itself is hard to see in this stretch today, where homes and overgrowth block the streambanks. It also runs a good 20 feet below the surrounding land, a typical East Bay arroyo with banks like cliffs. Except for a lovely little fenced-off area on Barry Place, distant glimpses are the closest you can get to the water.

By textbook standards, Sausal Creek has the most well-formed floodplain in Oakland. A floodplain forms wherever a stream runs out of downhill energy and has to drop the sediment it’s carrying. The elevation profile I made for Sausal Creek shows the floodplain portion.

This part of the stream is at grade, like the canyon above it and the delta below. It’s very stable. Neither fully erosive nor fully depositional, a stream in a floodplain has little it can do except rearrange the sediment.

Floodplain streams tend to turn snaky, meandering and migrating like the trickles you’ll see on a rainy windshield. They nip off bits of the bounding hillsides and gradually widen their corridor.

_{From Phil Stoffer’s geologycafe site}

A floodplain is a place where sediment languishes during its slow journey to the delta. A lot of that sediment enters the floodplain from the hills on either side. Here are three different views across the floodplain showing its flat floor and abrupt edges. Fruitvale Avenue at E. 27th Street:

Farther upstream at MacArthur Boulevard:

And still higher at Coloma Street.

For real detail, I always look for a digital elevation model or DEM. The DEM of the upper end of the floodplain shows how flat its floor is and how fragile the west side of the valley is. It also shows how much the stream itself is meandering within the straight floodplain valley.

Unlike the rocky flanks of Dimond Canyon, the hills lining Sausal Creek’s floodplain consist of sediment, ancient gravel beds that I’ve named the Fan. Time to look at the geologic map of the floodplain section.

The material of the Fan, labeled Qpaf on the map (“Quaternary Pleistocene alluvial fan”), is unconsolidated sand and gravel that’s prone to landslides on steep slopes. That scoop out of the hillside just left of center in the DEM image is the notorious McKillop slide, in progress for more than a century. Much of the crumpled land there was smoothed over and preserved as William Wood Park, but the adjoining slope to the south, which failed in 2006, is still a mess.

Two things about this stretch of Sausal Creek strike me as odd. The first is how straight the stream valley is here. Streams tend to curve, so if you find a naturally straight one you look for what’s forcing it to be that way.

My tentative theory, which I alluded to at the end of the previous post, is that the Fan was tilted up, not built up, along with the Piedmont bedrock block thanks to tectonic interactions along the Hayward fault. (In other words, the Fan is not really an alluvial fan but an uplifted relic of an older coastal plain.) I suppose that the stream was incised in a straight line by large flood events early in its history, then widened into the floodplain we see today. It’s easy to imagine big pulses of floodwater coming out of Dimond Canyon like a firehose, most likely after a temporary lake above the canyon, dammed up by an earthquake landslide, gives way. Such floods would rush in a straight line to the sea and set the stage for the floodplain that evolved.

The other odd thing is how closely the stream hugs the west side of the valley. So does Peralta Creek, just east of Sausal Creek. I think this may not be an accident. Glen Echo Creek, on the other side of the Fan, hugs the east side of its valley. Perhaps there is some subtle warping of the crust across central Oakland.

Sausal Creek’s fruited plain may be just a vale, but it’s a vale with some intriguing features.

This entry was posted on 28 October 2019 at 8:01 am and is filed under 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.