Archive for April, 2013

Leona Canyon

23 April 2013

leonacynsign

Leona Canyon Regional Open Space Reserve is an East Bay Regional Parks District property of some 290 acres that is entirely within the Oakland city boundary. It’s got rocks.

The canyon was cut by Rifle Range Branch, part of the Arroyo Viejo stream network. The branch joins Arroyo Viejo underneath I-580 at the turnoff to the zoo. The topography is rugged. I surmise that the rifle range that gave its name to the creek was here once upon a time, because it’s the sort of place where you could shoot a lot without disturbing the rest of the city.

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Here’s the geology of the same piece of ground.

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The pink “Jsv” is the same metavolcanic rock (Leona “rhyolite”) found in the Leona Quarry just to the west. The green units are the familiar sedimentary rocks of the Great Valley Sequence, tilted upward so that they get younger to the east. The units, in order of age, are the Knoxville Formation (KJk), the Joaquin Miller Formation (Kjm), the Oakland Conglomerate (Ko) and the Shephard Creek Formation (Ksc). You can see that the canyon is controlled by the faulted contact between pink and green.

OK! The creek is dammed at the base of the canyon, presumably just for flood or sediment control. Maybe the rifle range used to be here. Anyway, the creek is fairly level throughout the park, creating a nice bit of habitat.

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As you walk up the creek, it wanders along the contact between the two major rock units, so you’ll see a mixture of boulders in the creek bed. The Knoxville is a shale with some sandstone, not very distinguished, but near its base it includes some conglomerate and breccia: rocks made of pebbles and cobbles derived from the Leona keratophyre. This example is from the high end of the trail, in the upper left corner of the geologic map.

KJk-cgl

The reserve has two paths that lead up the canyon’s sides. The Pyrite Trail goes west through the metavolcanics. It’s shady and steep. I should note that I saw no signs of pyrite on it.

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Along this trail you’ll see the Leona metavolcanics, kinda ragged-looking stuff that’s been chewed up and spit out a few times since it was a volcanic island arc during the Late Jurassic.

Jsv-brec

There are nice views of the other side of the canyon, which is more open and chaparral-y.

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The trail up that side is called the Artemisia Trail. I’m not sure that either trail’s name means much. It passes a lot of this fine-grained sandstone.

Kjm-ss

Higher up, you get a good look at this big knob, which is a prominent part of the hills’ skyline as seen from the north. This view is from the south.

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There seem to be a few informal trails on it, and the view must be fantastic. But the Artemisia Trail offers superb views across the middle and south bay, too. I’ll be back.

San Leandro Creek (4)

16 April 2013

The airport area, to a land planner, probably seems like a blank canvas labeled “Raw Land.” But there’s stuff going on here.

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This is where the low end of San Leandro Creek, between Hegenberger and the Nimitz, goes through some significant transitions. Here’s the Google Maps view, followed by the geologic map. The photo above is from the corner of Leet Drive at Hegenberger.

SLcreekGmap

SLcreekmap

The pink area is artificial fill. “Qhb” stands for young basin deposits, which is the zone of sediment at the very bottom of the San Leandro alluvial fan just before you hit bay mud. The darker yellow bit at the top is a belt of levee deposits, where the creek once flowed perhaps thousands of years ago. The alluvial fan has a half-dozen of these splaying out from Lake Chabot, from Elmhurst Creek on the north down to southern San Leandro.

(By the way, the topographic base of this geologic map is quite out of date. Do any of you remember the drive-in theater shown here where Kitty Lane is today? And when was Dag Hammarskjold School, so righteously named, ripped out and replaced with a public-storage joint?)

Looking south (upstream) from Hegenberger, the creek appears almost natural, with its floodplain close-hemmed by levees. At least, it has mud on the bottom and vegetation growing in it.

SLcreek1

This stretch of the creek is fenced off and inaccessible. From 98th Avenue—more precisely, looking downstream from behind the Wendy’s at the head of Bigge Street—it looks even more bucolic.

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Between these two points is the original Bay shoreline, a gentle transition from low grassland to high marsh that is almost totally gone from the Bay today. This part of the creek is the nearest it gets to being natural. Upstream from 98th the entire creek is walled in.

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This last shot is from the spot where the power line crosses the creek; as usual the land beneath is an informal public park. A hole in the fence gives access to the creek, but the going looks tough, and besides the dogs here are numerous and excitable.

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The creek forms the boundary between San Leandro and Oakland from here to the railroad tracks. From the tracks to 580, San Leandro spills well north of the creek because, like the village of Temescal, the city began as a town right on the creek.

Central Reservoir

8 April 2013

Central Reservoir is operated by EBMUD, but it’s much older. It’s the weird-looking steel-covered field north of Sausal Creek. This is a view looking over the reservoir from Ardley Avenue toward the hills.

centralrestop

That’s the Altenheim on the left, across I-580, and of course the LDS temple with Redwood Peak behind it.

The reservoir was built in 1910 by the People’s Water Company, which took the existing valley of a Sausal Creek tributary, hollowed out the top of its watershed and made an earthen dam. Later EBMUD assumed control of it and upgraded things considerably. However, landslides plagued the steep west bank of Sausal Creek directly east of the reservoir starting in the 1930s.

The latest set of slides, in 2006, led to a tangle of lawsuits initially aimed at EBMUD and blaming leakage from the reservoir. The lawsuits were consolidated and went to a jury trial in 2012, with Alameda County as the main defendant and the damaged land owners (two homeowners and a church) as the remaining plaintiffs. The jury found for the County. None of the media that announced the lawsuit bothered to report the outcome, and the City of Oakland hasn’t bothered to clear EBMUD’s name, but the jury dispensed justice as designed.

For 17 MB of geotechnical detail, see EBMUD’s Central Reservoir Seismic Final Report, issued in 2008. As far as engineers can tell, even the Big One on the Hayward fault won’t break the dam. But if I lived downstream, I’d keep a close eye on the dam after a truly major quake and be ready to relocate. And in the aftermath, that emergency water supply may save our lives.


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