Debris Dams of the San Gabriel Mountains

[Sean]

Let's devote this thread to documenting all the debris dams we find in the local mountains. To avoid repetition, please include some details about each dam in order to distinguish it from all the others. At minimum, give the name of the canyon in which it resides. Also, these structures typically have identifiers stamped into the concrete somewhere. Please add a picture of the stamp if possible.

I'll begin with a few that I checked out today in El Prieto Canyon. There are more in there that I didn't document.

Stamp: USFS 1972 ?

Stamp: USFS 1968 T1

Stamp: USFS 1971 ?

Stamp: USFS 1971 M5 LACFCD

Stamp: USFS 1969 M4 LACFCD

Stamp: USFS 1974 M13

[dima]

There're 6 debris dams down there?? I was never too clear on what these are for. Enlighten me?

[Sean]

There're 6 debris dams down there?? I was never too clear on what these are for. Enlighten me?

There are way more than six. These are the only ones I had time to document because they were easily photographed from the trail. The trail bypasses several more in El Prieto, and it would take some scrambling to get the stamps on those.

They are built for flood control. They catch and hold loose debris, which then absorbs water flow, as opposed to bedrock which doesn't. This mechanism slows the outflow pouring into the main branch of the watershed, in this case the Arroyo Seco, so that the channel through the city doesn't fail during heavy rain events. The Arroyo Seco historically caused major flooding in Pasadena. So they built a ton of these debris dams in the major tributaries, plus Devil's Gate and the Brown Mtn. dams in the main branch.