Archive for the ‘oakland water’ Category

Two lessons about floods

19 October 2015

As we anticipate the strong possibility of heavy El Niño rains, my attention will be on Oakland’s streams this winter. Last week parts of southern California were hit by “thousand-year” rainfall events, cloudbursts that washed thick sheets of mud over roads and properties. We can expect such things here too, in any given thousand-year period.

Arroyo Viejo, the stream that crosses Knowland Park, offers two lessons about floods. The scene below is at the northern edge of the park, looking upstream: a streambed piled with boulders, some as large as sofas. (All photos 800 px)


Notice: these rocks have been tumbled by the stream. How much water would it take to do that? Let’s make a rough, arm-waving estimate.

The rainfall in last week’s cloudburst was almost 4 inches in one hour. Had it fallen on the watershed of Arroyo Viejo above this point — say, half a square kilometer — it would represent an input of roughly 30 cubic meters of rainwater every second.

Picture in your mind that volume of water — no, it would be mud and therefore that much greater — funnelled through this narrow valley. Do a little geometry and it’s easy to see the floodwater would be well above the tops of the boulders.

Hidden in plain sight in this photo, then, is a single hour of tumult that might have happened a thousand years ago or five hundered years ago — or perhaps during the dreadful winter of 1861-62, when it rained for 43 straight days and much of the Central Valley became a lake.

The lesson is that most of geology’s hard work gets done in rare spurts of extraordinary activity.

Okay, the second lesson is hidden in these rocks. All of them, like this boulder as tall as me, are made of conglomerate.


These rocks, assigned to the Knoxville Formation of Late Jurassic to Early Cretaceous age, were laid down by ancient floods in a nearshore or terrestrial setting. I’ll show you three different specimens. Notice the large clasts and the fine-grained matrix that surrounds them.


This boulder displays a wide range of clast sizes. It was probably laid down by what’s called a hyperpycnal flow, a slurry of sediment that carries everything along with it. We’ve watched them happen offshore in Monterey Canyon. Here’s another example.


Instead of an underwater landslide, as seen in the first specimen, this represents something gentler and more organized, like a mudflow, or like the mudslides we saw in the news. The clasts are aligned with the current that carried them here.

The key observation in both cases is that the large clasts are floating in the matrix. In geologist’s terms, they are matrix-supported conglomerates.

Then we have this.


Here’s a beautiful clast-supported conglomerate. It represents a clean bed of well-rounded cobbles, all touching each other, like you’d see in a rushing stream or a rocky beach, nicely infiltrated with clean silt or clay after it was laid down.

None of these stones were made by ordinary sediment wafting down streams during ordinary rainy seasons. They were assembled by floods of all sizes.

Triple creek junction

17 March 2015

A while ago I featured the casting ponds at McCrea Memorial Park, in the valley of upper Lion Creek. I didn’t poke further downstream at the time, but since then I have. The creek runs alongside a pair of small concrete “trout ponds,” cunningly made with cobble-lined runnels that would send a lifegiving trickle through them if there was enough streamflow, which there isn’t, to sustain fish, which there aren’t any of.


The lower pond did have enough water in it to interest a mallard couple. The male kept watch on me as the female gorged on duckweed.

To the left of that photo, just over a low ridge and behind a fence, the iron-stained waters of Sulfur Mine creek exit a pipe.


Farther down, it joins Lion Creek and the combined watercourse enters a tunnel beneath the Warren Freeway. I haven’t gone into it, but people clearly do.


If you cross the freeway on the little-used pedestrian overpass, you might expect to find the creek on the other side. Instead you’ll see Horseshoe Creek coming out of its canyon in Leona Heights Park and entering its own final conduit.


Where the two creeks meet must once have been a cheerful place. Today it must be a black and dismal one, visited only by rare daredevils. If the tunnels aren’t screened, perhaps they can crawl all the way down to where Lion Creek reemerges, at Lake Aliso at Mills College, in a wretched feat of urban spelunking.

Seminary Creek at Mills College

14 February 2015

The third creek running through Mills College is Seminary Creek. It gets its name not just from Mills, but from the Beulah Heights district that forms its headwaters. On the 1897 USGS topo map it’s the dashed blue line, signifying an intermittent stream, running due south to East Creek (now named Lion Creek).


Nowadays it’s almost entirely culverted. Not only that, it’s been kidnapped! The Oakland Museum of California’s creek map shows the creek as being redirected away from Lion Creek. Today it runs just north of Seminary Avenue to a channel called East Creek Slough.


What glory the creek still has today is evident only on the Mills College grounds. It ducks between lobes 6 (Maxwell Park) and 7 (Mills) of the Fan next to MacArthur Boulevard, but a close look at the topography suggests that a landslide or earthquake could easily have made it spill to the east of lobe 7 (past the triangle marked “30” on the stream map). Be all that as it may, the creek daylights just where MacArthur curves west.


It then wanders a couple hundred yards through a nice quiet forest, mostly eucalyptus. Other than putting a parking lot over half its course, the college appears to have left Seminary Creek alone.


When it hits MacArthur, at 57th Avenue, Seminary Creek disappears. Here’s the last sight of it, from the MacArthur side. Here’s where the stream got kidnapped.


I’ve taken photos of the creek farther downstream, but it’s a nondescript ditch and now I feel sorry for it. So I’ll spare you.

Interestingly, the old map showed the creek as intermittent, but I’ve never seen it dry even after our three-years drought. As I mentioned about Chimes Creek, I think undergrounding Seminary Creek has kept it from evaporating.


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