One of the largest watersheds in Southern California, the San Gabriel River ends its 58 mile journey here at the Pacific Ocean between the borders of Los Angeles and Orange Counties. The river cuts through rocks in the San Gabriel Mountains dating over a billion years old, carrying sediments all the way here to Long/Seal Beach. The river today mostly consists of street runoff and snowmelt in the winter. On the western side of this bridge, there is a sidewalk where you can answer all the needed questions to get credit for this earthcache.
The closer the water gets to the ocean, the slower it flows which allows finer particles such as silt and sand to be deposited near the mouth of rivers, like the San Gabriel. The San Gabriel River begins its 58 mile journey in the San Gabriel Mountains, near Pearblossom. Other smaller creeks and tributaries also flow into the San Gabriel River, making it much larger. These creeks include Walnut Creek and San Jose Creek which go deep into the San Gabriel Mountains. These mountains have similar geology, both with strike-strip motion. The rivers in these mountains carry dissolved chemicals and sediments to which are carried all the way to the ocean at this point. At the snowcapped peaks of the San Gabriel Mountains, the metamorphic rocks are dated to be 1.7 billion years old, while near the base of the mountains the granite rocks are dated at 75 million years old.
There are a few things to note when talking about sediment deposition from rivers. One thing, is that it depends on a few factors. Speed of the river (which varies quite a lot), size of the particle, and the calmness of the ocean. Let's discuss the speed of the river. The fastest part of the river is located in the middle, just below the surface. A common misconception is that middle of the river at its surface is the fastest, but due to wind this slows it down. The edges and shorelines of the river are always the slowest.
Rivers can deposit different sediment. Sand, silt, and clay are the most common, but during large rain storms and floods, rivers can move large rocks and even boulders. Silt is the smallest particle that rivers can carry. See the chart below for the particle and their given sizes. A thing to keep in mind is that the smaller the particle is, the calmer the water needs in order for it to be deposited. It's one of the reasons why sand makes up beaches, and not silt or clay. Silt and clay are suspended in the ocean waves until they are carried enough out to see where the water is calmer, and are deposited on the ocean floor.
During storm seasons (November - March), the river can flow quite fast near the mouth, making deposition difficult and only occurring offshore. Because the mouth is located in a highly populated area, much of the river is made of concrete basins rather than a natural river valley like Newport Back Bay. A concrete basin allows water to flow more quickly, to reduce flooding upstream. It also allows easier control methods to reduce pollution as it is much easier to see where the point source is coming. However, during flooding, natural rivers push larger sediments, such as boulders, to the sides creating natural levees and protecting the river for years to come. Most of the time, the river doesn't flood and brush begins to grow on these levees, soaking up water during rainy seasons. Plants protect the rivers in addition to the natural levees. With the Santa Ana being a natural river, but altered with concrete basins, flooding will occur more frequently especially in low lying areas near its terminus.
Works Cited
https://geomaps.wr.usgs.gov/parks/rxmin/sediment.html
https://en.wikipedia.org/wiki/San_Gabriel_River_(California)
http://www.geography.learnontheinternet.co.uk/topics/river_deposition.html
https://scienceviews.com/geology/sedimentary.html
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL OF THE QUESTIONS BELOW. You can contact me through the geocaching message center or email to submit your answers. Any incorrect answers may result in a clarification response.
Note: You will only receive an email from me if I need clarification on your answers. You are free to log the cache as a find whenever you like.
1. "San Gabriel River Terminus" on the first line of your email.
2. Take a photo of you (or your signature item if you don't want to show your face) with the San Gabriel River in the foregrond and the Pacific Ocean in the background. In other words, your back must be facing the railing and you must be facing the road so that both the river and ocean are behind you in your photo.
3. As you walk to the above coordinates, look for any signs of levees along the banks of the river. If they exist, tell me: are they natural or man made? And how can you tell?
4. Look at the river towards the Pacific Ocean. Describe the conditions of the water. How fast is it moving? Is it choppy or calm? What direction is it moving? Etcetera.
5. Describe the water clarity and quality. Can you see any sediments? If so, according to the sediment chart, what can you see? What are the sizes or shapes? Explain.
6. As you walk back to your car, look for signs where the city has prepared itself in case of a massive flood. Do you see anything that would help prevent a flood (or tsunami) in a major storm?