Archive for the ‘oakland water’ Category

Lake Aliso

30 December 2014

Mills College occupies a geologically interesting part of town. It owes its stimulating geomorphology to the confluence of three streams under the influence of the Hayward fault. I plan to write several posts about it. Here’s the segment of the geologic map that includes the campus.


The main strand of the fault runs just left of the “Jb” symbol. The narrow lobe of tan, symbolizing Pleistocene alluvium, is where Lion Creek turns from its southward course and cuts across a low ridge of Jurassic basalt (Jb) to cross the fault. I have to say that I haven’t yet found any basalt there, so treat the map with caution. After every large earthquake, whenever and wherever the ground is uplifted the creek, momentarily dammed, gathers its strength and cuts its way through to maintain its right of way. But a flat spot in the streamcourse persists above the fault trace, and there may be a tectonic element at play there too, downdropping the spot in a sag basin. In any case, that wet spot is where the college’s administrators erected a dam to create Lake Aliso, a picturesque basin that was also useful (1) as a water supply for landscaping purposes and (2) for regulating the creek in an attractive state of flow, neither flood nor trickle, as it traverses the campus.

Old photos show the lake as a fine place for boating and pageants, but sediment has inevitably filled it in. Today it’s trying to return to marsh, and from there it aims to retire as a nice meadow.


But we made the lake, and we can maintain it with enough money and machinery. Here’s Lion Creek, such as it was, at the lake’s inlet, which must date from the building of freeways I-580 and Warren.


My visit was a few weeks before the December rains but after November’s whistle-wetting, so the water was cloudy with fresh sediment and possibly some of that ugly runoff from the old sulfur mine. Right now Lion Creek should be closer to roaring.

The other end of the lake is an earthen dam, including this spillway.


It demonstrates one of the basics of managing streams of any size: If you block a stream, it will silt up its bed on the high end and start eroding its bed on the low end. Another way to think of it is that when we mess with a stream, it usually backfires in the long run. The guidance of a licensed geologist with some expertise in hydrology can help forestall the inevitable.

There is some loose rock around, most of it looking like this.


Although they may just be landfill, I assign these stones to the “Jsv” unit—the metamorphosed volcanic rocks that make up the high hills.

After Oakland’s Big One, expect a rush of water

8 September 2014

In the days since the South Napa earthquake of 24 August, the people and press appear to be astonished as the local streams have filled with water. The Chronicle published a good summary yesterday. But this always happens with a decent-sized earthquake. It will happen here. You can expect to see this stretch of Arroyo Viejo, in the middle of Hegenberger Expressway, full of freshly released groundwater.


There were widespread reports of this kind in 1857, after the the great Fort Tejon earthquake of 9 January:

On a range of hills, about fifteen miles from the coast, in the district of San Fernando, we understand that a surveying party have discovered quite a large stream making out of the mountain and down a cañon, where, to their knowledge and complete satisfaction, not to say to their sorrow, no water was running or could be found previous to the earthquake. By the letter from Tejón, it will be seen that a similar circumstance occurred in that vicinity. Los Angeles Star, 17 January 1857

Just back of my camp was the dry bed of a stream, where in heavy rains water had at one time run; in this bed two weeks before I had sunk a well some 20 feet hoping to find water, but at that depth the earth was so dry I gave it up as fruitless. Two days after the first or heavy shock a little stream of muddy water was running by my camp which continued to increase each day, until when we moved was quite a little rivulet: no doubt the result of some new fissure in the mountain. Letter of W. E. Greenwell, U.S. Coast Survey, 24 February 1857

The effect upon some of the artesian wells in this neighborhood was remarkable: for a moment the water ceased to flow from the pipes, and then gushed out in greater volume and with more power than usual; we have heard that the channels of other wells, which had become obstructed, and ceased to discharge water, have become re-opened and the subterranean current is now flowing out from the orifice. San Jose Telegraph, 13 January 1857

The water goes away after a few weeks. UCB geologist Michael Manga explained the phenomenon in a talk I attended in May of last year: The shaking settles the bedrock, which in turn forces the groundwater it contains upward. In effect, the rock’s permeability in the vertical direction increases as a result of basement consolidation. This is the same basic mechanism that creates quicksand. Studies after the 1999 Taiwan (Chi-Chi) earthquake showed that this water would be replaced in about 140 years—which is coincidentally the average interval between large quakes on the Hayward fault in the last thousand years. The effect takes place within a rupture length of the fault—that accounts for the response of wells in San Jose to a Southern California earthquake whose rupture ran about 360 kilometers, from Parkfield to Cajon Pass.

Oakland’s droughtproof lake

5 September 2014

I’ve been walking around town a lot this year, and our perennial streams still have running water even in the third year of severe drought. If we still had any natural lakes, I think they’d be suffering, just like our reservoirs. But we don’t. Whatever else happens, we have Lake Merritt.


Lake Merritt isn’t like other lakes: it’s an arm of the Bay. So let’s relish our luck and make the most of our inexhaustible, droughtproof “lake.”


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