Lake Aliso

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.

mills-college-geomap

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.

lake-aliso

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.

lake-aliso-inlet

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.

lake-aliso-outlet

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.

lake-aliso-rock

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

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5 Responses to “Lake Aliso”

  1. artisancrafts Says:

    There is a trail that runs from the rear of the art museum and the swimming pool. It was the rail bed for the Courtland route.
    Along the way, you’ll find a bronze survey marker from the city of Oakland. Very near that spot is a solid rock stream bed that is made of some of most pretty rock you’ll see around these parts. If it isn’t a huge chunk of blueschist, then I no longer like blueschist, I like this stuff instead.
    If you’re already written about this rather startling feature, then oops. If you have trouble finding the spot, lemme know!
    Thanks. Can you explain how this rock is formed (if you have not already done so.), and explain how it ended up here?
    I love this micro-area, there are so many things to find!
    Erich

  2. Andrew Says:

    I’ll check it out as soon as I can.

  3. Kevin Says:

    I know this is somewhat out of the area of your blog, but your mention of the lake silting up reminded me of one in my area, Searsville Lake: http://www.beyondsearsvilledam.org/Beyond_Searsville_Dam/Home.html

  4. Leslie Ozawa Says:

    This message is for andrew alden.
    Links to photos are broken on How to Look at Rock webpage on the About.com website. I emailed the ‘gethelp@aboutguide.com’ but got a non-deliverable message.
    http://geology.about.com/od/rocks/tp/rocks101.htm
    I hope the problem can be easily fixed.
    Thanks for providing content on the website.

    Les

  5. Andrew Says:

    Les, About.com cut me off their roster six months ago. There’s nothing I can do to my content, which they have licensed.

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