Oakland Geology

San Leandro Creek supplies almost all of the water in Lake Chabot. Here’s where it enters the reservoir, at its farthest eastern end.

San Leandro Creek is one of only three streams that cross the East Bay hills. The other two are San Lorenzo Creek, which cuts through Castro Valley and Hayward, and Alameda Creek, which traverses Niles and Fremont.

Since Anthony Chabot built the dam in the 1870s, the creek has dumped sediment and filled in about a mile of its course, creating new level land until today it’s slowly encroaching into the lake itself. Willow Park Golf Course is built on that land.

Not much sediment comes into the lake today, now that Upper San Leandro Reservoir has dammed the creek about four miles upstream. The round pads of aquatic vegetation flank the stream channel, which runs between low, muddy banks that were underwater when I visited last month. I think the vegetation traps sediment, which supports more vegetation, so that the patches tend to grow into a circular shape, reminiscent of bacterial colonies on a petri dish.

The first two ridges in this photo are underlain by the Joaquin Miller Formation, which extends along the whole south shore of the lake between here and the park headquarters. It’s mostly shale with a few large sandstone beds, tilted steeply upward. The farther ridges are all Oakland Conglomerate, and they stand maybe 50 meters higher. They look more sparsely vegetated, which may reflect the bedrock but might just as easily be due to historic land uses.

This entry was posted on 25 July 2010 at 3:36 pm and is filed under Oakland streams and water. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.