Oakland Geology

Oakland has several major, permanent streams crossing it from the hills to the Bay. Then it has Lake Merritt, formerly known as San Antonio Slough — an arm of the sea extending more than a mile inland from the shore.

What makes it so exceptional?

I have a theory, based on the last million years or so of geologic history plus some of the latest research.

First of all, we need to ignore the Lake Merritt we know today:

. . . and think of Oakland as it originally existed. This is an excerpt from the “Bache map” of 1857, a survey of the waters surrounding the newborn city of Oakland and its neighboring town of Brooklyn painstakingly made by the U.S. Coast Survey. It covers the same area as the Google Earth clip above. It’s a fat 1200-pixel image worth zooming in on (or study the full-size scan from Wikipedia).

“San Antonio Creek” was the inlet that led to the existing landing at Brooklyn. It had a central channel, just a couple hundred yards wide, that was deep enough for ships, and the rest was tidal mudflats or treacherous shallows. The slough extending to the north — today’s Lake Merritt — had strong tidal currents and a very shallow mouth. Small craft could use it when the tide was high, and duck hunters were a common presence there, but for serious commerce it was useless, and Oakland’s landing at the foot of Broadway was little better.

Back then, San Antonio Slough had a wider mouth lined with wetlands, with terraces roughly 25 feet high on either side. Later the mouth got filled in leaving the narrow passage we know today . . .

. . . but if you look for it, for instance down 10th Street past the museum and auditorium, you can get a sense of its original width.

My theory starts with taking the mind back into recent geologic history — the dozens of ice ages that have occurred regularly for the last 2-plus million years. When the ice caps were at their largest, the sea sat hundreds of feet lower than today. Except for the Golden Gate itself, the whole Bay was dry land, and all of our creeks ran out far beyond today’s shoreline to join the combined Sacramento-San Joaquin River. Today’s Lake Merritt, then, is a drowned stream valley — a term east coast geologists know well, but seldom used around here.

For clarity’s sake I will use the name Merritt Creek for the stream that occupied that valley during glacial times. Glen Echo Creek ran into Merritt Creek down a swale where the north arm of Lake Merritt sits today.

The eastern arm of today’s lake was where three creeks joined: Pleasant Valley, Wildwood and Indian Gulch (Trestle Glen) Creeks. You know, let’s call the drowned valley Pleasant Valley, because it surely was one. The late Pleistocene creatures and vegetation there I will leave to your imagination.

Three more smaller streams also drained into Merritt Creek: “Kaiser Creek” at 20th Street, “Adams Point Creek” at Perkins Street and Park Boulevard Creek at the E. 18th Street landing.

Here they all are on the watershed maps from the Alameda Country Flood Control District.

And if you adjust this map in your mind by subtracting the sea, Merritt Creek also received input from 14th Avenue and 23rd Avenue Creeks (that is, the rest of San Antonio Creek).

My argument is that Merritt Creek is a drowned valley today, instead of an ordinary creek like the rest of Oakland’s streams, because it cut down deeper than other creeks. I can cite three reasons for that.

First, Merritt Creek had the largest watershed between San Pablo and San Leandro, thus it had the greatest water-gathering power in the area — especially during glacial times. And as the watershed map shows, the stream network is well organized, capable of delivering stormwater in a big flush. It didn’t dribble across a wide coastal plain like Temescal and Sausal Creeks on either side. Whereas those creeks spread out their floodwaters on the plain and slowed their flows (depositing their sediment across the landscape), Merritt Creek was confined between elevated banks and couldn’t slow down. It was better equipped to cut into the exposed floor of the Bay.

Second, Merritt Creek drained a large area of hard bedrock: the Franciscan sandstone, shown in blue on the geologic map, that underlies the hills of Piedmont. I argue that this substrate didn’t generate as much mud or clay as its neighbors and made the stream network less prone to clogging.

Third, unlike Oakland’s other major streams, Merritt Creek’s watershed didn’t cross the Hayward fault and was not affected by it. This is an intricate subject I plan to address in future posts as well as my book. Briefly, the fault messes with streams as its sides slip past each other. Headwaters in the hills get slowly cut off from their downstream reaches. Streams get stretched and snap, interrupting their natural evolution into well-organized networks like Merritt Creek’s. The head of one stream gets grafted onto the stem of another stream, and the transportation of sediment from hill to bay — the basic function of streams — is stymied and randomized.

Maybe this argument is easier to read in a simple image, a shaded digital elevation model of central Oakland. The fault line is obvious, as is the integrity of Merritt Creek. Temescal and Sausal Creeks reach around Merritt Creek’s drainage, like hands holding a bowl, and cross the fault with disruptions you can explore on the AC Flood Control District site.

Another more scientifically phrased argument was just published in the journal Earth Surface Processes and Landforms. The paper is based on the example of the Dead Sea, where human intervention has been lowering the world’s saltiest lake. A team of geologists took that as an analog of the glacial cycle and asked how the streams feeding the Dead Sea have responded. The bigger, wetter streams cut down into the land, keeping up their deliveries of sediment as the water recedes, while the piddly streams give up and stay behind. Reading the abstract, I immediately thought of our creeks and the exceptional one whose drowned valley is, for the moment, our little mediterranean sea, our miniature San Francisco Bay, named Lake Merritt.

You know how the Pleistocene was, full of large beasts that have slouched off into extinction: mastodons, giant ground sloths, sabertooth cats, dire wolves and so on. There were monsters around then.

And today we have three monsters around the lake. Have you seen them? The newest one is named Makkeweks, inspired by Ohlone stories, and lives in Snow Park.

Makkeweks joins the newly restored Mid-Century Monster (here as seen in 2005) . . .

. . . and the original, the one and only Fairyland Dragon.

Think of the Pleistocene when you visit them.

This entry was posted on 16 September 2019 at 8:01 am and is filed under Oakland geology puzzles, 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.