Archive for the ‘oakland water’ Category

Chimes Creek headwaters

1 February 2016

I’ve mentioned how tempting the uppermost catchment of Chimes Creek looks, perched above the Leona Quarry scar:

chimesCktop

Finally got up there last week. Access is difficult and not for casual visits.

Here’s the valley in Google Earth, looking obliquely at it. At first glance it looks natural, but it’s heavily engineered.

Chimeshead-view

I’ll show photos going from top to bottom, between the two dots on the above image. The valley above the upper dot was filled in by the Ridgemont developers, using material cut from the ridge to its left. This is the view downhill from the position of the upper dot.

Chimeshead-1

The floor of the valley here is crisscrossed with concrete ditches, which converge about a third of the way down the transect at a culvert. Along the way you pass a large outcrop. All the rock here consists of the Leona “rhyolite.” The outcrops are tempting, but the slope is steep and treacherous.

Chimeshead-2

Here’s the culvert, possibly the only one in Oakland without graffiti, joined from the right side by the concrete ditch. I think it must carry runoff from the Ridgemont streets. But what’s that ugly orange?

Chimeshead-outlet

Why, it’s acid drainage from one particular part of the subsurface here. I would love to see the geotechnical reports from the time they built this development.

Chimeshead-pipes

This is one of many places where I’ve seen “yellowboy” in the Leona; the most notorious is the old sulfur mine, of course, and I noted another last week. By now I think that every excavation in this rock unit, old or new, should be treated as a potential hazard.

The next couple hundred feet downhill from here is a lovely tree-shaded, undisturbed steep cascade over large boulders. Here are just two of them.

Chimeshead-3

This beautiful rock is extensively slickensided (polished by underground movements) and coated with the iron hydroxide minerals that result from natural weathering. It is not stained orange by the pollution from upstream; in fact the water at this point is only slightly milky. It’s very much like the rocks in the Redwood Road boulder pile.

Chimeshead-4

And what to say about this one? It’s an unusual lithology within the Leona. My impression just from eyeballing it was that it’s an autobreccia — a ground-up body of lava or tuff consisting of lumps of the original rock in a matrix of pulverized (and relithified) rock. But that’s not the only possibility.

Another outcrop up on the valley wall exposes a slickenside that covers a good square meter.

Chimeshead-5

At the base of the cascade is more engineered land, a small catchment housing a screened culvert entrance. Chimes Creek is trapped here and conveyed beneath the old quarry and across the freeway to emerge in the Millsmont neighborhood. As I say, it’s engineered land, but it’s planted with trees and rather pleasant. It also catches runaway rocks before they can take out a townhome down below.

Chimeshead-6

The view up from the lip of the catchment shows the two outcrops and the shape of the land.

Chimeshead-7

I can’t wait to return for a closer look, though with so many other places to see it might be a while.

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)

A-viejo-in-K-park

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.

Knox-in-A-viejo1

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.

Knox-in-A-viejo2

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.

Knox-in-A-viejo3

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.

Knox-in-A-viejo4

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.

leonack-troutponds

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.

sulfurmineck-outlet

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.

sulfurmine-lion-ck-junction

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.

horseshoeck-mouth

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.


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