While the San Gabriel River continues north for several more miles, access to the river via turnouts on Highway 39 (San Gabriel Canyon Road) is poor, making this turnout the best to get out and see the river. This is the view looking east across the canyon towards the river. The line of Sycamores marks the edge of the river.
From the highway, follow the dirt road into the canyon. After a short while, there is a fork in the road; you can go straight (north), or to the right (east). Take the right path and you will come to another fork in the road. If you follow the path to the right, you will cross the San Gabriel Fault (poor exposure). To continue on to the river, stay on the main path that veers to the left.
Side Channel
The San Gabriel River has one main channel and several side channels. During periods of high rainfall, most of the canyon floods. As the water recedes, side channels can form, only to dry up as rainfall ceases.
This photo shows the boulder-strewn side-channel adjacent to the main channel slightly to the east.
In the mountains, clasts are closer to their source. This means that they have not traveled a great distance, nor have they been subjected to as much erosion and transport as those further downstream. As a result, clasts in the upper portions of rivers are
- larger (cobbles and boulders)- less rounded and less spherical- poorly sorted
It takes energy to move clasts. The larger the particle, the heavier it is, so the more energy is needed to move it. Take a look at the image above again. Note how many boulders are in the photo. A lot, right? Think about how much water - and how fast that water would need to be moving - in order to push those boulders downstream even a few feet.
Also visible in the photo are smaller clasts. Geologists use a variety of tools to estimate grain sizes. One of the simpler methods is to use a ruler to measure how wide it is. Another is to use a sediment sorter. These come in a variety of sizes, but consist of a round pan with a metal mesh of varying sizes inside. The most common pan size is 12 inches in diameter and 3 inches high. Considering you might need several pans, these sorters can be cumbersome to use in the field. The tool I find the most useful is a meter quadrant. It's basically a square made of PVC pipe with string grids on the inside.
This video shows the river channel, and how I use the quadrant.
The photo below shows the quadrat placed over a random area of the riverbed. Each side of the quadrant is 0.5 meters, with a 10 cm grid spacing inside. By placing it on the ground I can get an idea of how big each clast is within the quadrant. Note that all of the clasts inside the quadrant are smaller than 0.5 meters in diameter and that nearly all of the clasts are smaller than 10 cm in diameter as well.
Let's take a look at the area inside four of the grids, denoted by the yellow square in the image above. Remember, each side of the orange string squares is 10 cm.
There are several clasts to look at.
The first, #1, is a piece of Lowe Granodiorite. It is just a bit smaller than the 10 cm grid, approximately 7 cm or so. Using the Wentworth classification, this would place its size as a "cobble".
The second is a piece of gneiss. It is far smaller than the 10 cm grid - approximately 3 cm in length. Again, using the Wentworth classification, this would place it in the "coarse pebble" category.
Next, the third clast is another piece of Lowe Granodiorite. It's even smaller - maybe 1 1/2 cm in length. This would place it as a "medium pebble'.
Finally, there are even smaller fragments scattered about. These clasts range in size from medium to coarse sand.
Now, let's look at the shapes. None of the clasts are truly round, but many of the edges have been rounded off as a result of the moving water in the river. If you compare it to the shape chart, they would best be classified as "sub-angular".
Finally, note the distribution of grain sizes. There isn't just one size in the river bed, but a variety ranging from medium sands all the way up to boulders. This is an example of "poor sorting"
Sub-angular shapes, large clast size, and poor sorting are all characteristics of sediments found in river beds within the mountains, as they all indicate that the sediment hasn't traveled too far from the source.
Main Channel
Like most mountain rivers, the San Gabriel River in this location isn't very big. Its approximately 20 feet wide, and with a maximum depth of a few feet.
In this video you can see how fast the water is moving, Also, at one point I note the shape of the rocks are "rounded" - I goofed! I meant to say subangular. Apologies for the poor audio.
San Gabriel River, looking upstream (north). Note the large boulders on either side of the riverbank and within the channel.
In this view of the San Gabriel River, looking downstream (south), you can see the "V" shape of the channel, in addition to more boulders. Signs of human interference are everywhere, ranging from trash to graffiti-covered rocks.
