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Stop #2: Switzer Picnic Area

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Switzer Picnic Area is just off of Angeles Crest Highway and is a popular spot for local hikers as one can walk a short distance down the trail to Switzer Falls, one of the few (semi) perennial falls in Los Angeles. The picnic area is located in Colby Canyon next to the Arroyo Seco. Here, the arroyo is V shaped – an indication that uplift is actively occurring and that the river is down cutting through the mountains, forming a “V” shape as it does. Rivers located in mountains are often narrow, shallow, and fast moving. They have a smaller channel, which means a smaller discharge (volume of water flowing in the stream) and a higher velocity (the water moves faster). The higher velocity means higher energy, which means the river has a greater competence (maximum particle size the water can move). So, what happens in mountain streams is that the fast moving water moves the smaller grain sizes downstream and the larger particles such as boulders remain in the riverbed.

The view looking west along the Arroyo Seco in Colby Canyon. Note the V-shape of the canyon and the large clast size in the stream bed. Clast size is bimodal (two modes): Large, cobbles up to boulders, and small, coarse sands and fine gravels. Clasts are comprised of mostly of granodiorite.

Close up of the streambed sediments.

Here, the Arroyo Seco has cut deep into the granodiorite bedrock, forming a smooth rock surface on the right (east), and alluvium full of boulders on the left (west). The Gabrielino Trail is just above the alluvium and is located on a river terrace. The terrace formed over time when the river flooded, leaving behind layers of sediment as the flood waters receeded. The sediments on the terrace are mostly fine to medium sands.