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Rivers are more than a beautiful blue ribbon of water threading its way across the landscape. Rivers move excess water out of the mountains to replenish the ocean. Rivers also help to move broken bits of rock – called sediments – out of the mountains to be deposited in various locations on the path to the ocean. The area over which a river drains is called a watershed and includes the main river and any tributaries that enter into it. Some watersheds, like that of the Mississippi River, are huge, while others are small. The Los Angeles River watershed drains 834 square miles, covering the San Fernando Valley, the western San Gabriel Valley, and parts of the Santa Susana and San Gabriel Mountains. This trip will follow one of the tributaries of the Los Angeles River, the Arroyo Seco, from its headlands in the San Gabriel Mountains to its confluence with the Los Angeles River. In addition, we will look at two areas of the Los Angeles River that are being revitalized.

There are three possible paths precipitation can take once it falls from the clouds:

  • Infiltration: water gets absorbed into the ground
  • Transpiration: water gets transported through plants from the roots to the leaves.
  • Runoff: water runs over the ground’s surface and collects in low-lying areas.

The path we are interested in is the last one, runoff. Water runs off the steep slopes in the mountains and into low-lying areas before it begins to head downstream. As it does so, the water cuts into the rocks to create “V” shaped valleys. The shape of the valley forms from two actions. The first is simply the erosive force of the water acting upon the rocks beneath them. The second has to do with elevation. Water will always try to maintain a flat, level surface. The water in the river high in the mountains is connected with to the water in the ocean (aka base level). Thus, the water in the higher elevation will try to lower itself by down cutting through the rocks to reach the same elevation as the sea, and produces the steep “V” shape of the mountain valleys as it does.

These mountain rivers are also called “upper valley” rivers. Upper valley rivers are typically smaller in width and the water moves more quickly through them. A smaller channel means a smaller discharge (volume of water flowing in the stream); the steeper slope of the channel means the water is moving at a higher velocity. The faster the water moves the more energy it has, and more energy means that the water can move larger particle sizes. This is called the river’s competence – the maximum particle size a river can move (e.g., a boulder vs. a sand grain). Sediment travelling in a river is called the river’s load. Competence is related to a stream’s capacity – the maximum amount of sediment (load) it can move. Thus, upper valley rivers, like those at stops 1 – 3, have “V” shaped valleys, have smaller channels, and the water moves at a higher velocity. Their riverbeds typically contain larger clasts (cobbles through boulders) while smaller grain sizes (clays through sands) are moved downstream.

As a river transition from mountains to flatlands, the stream velocity decreases. The decreased velocity means a decrease in the river’s competence – the river can no longer transport the larger grain sizes and the load (sediments) get deposited. Features such as alluvial fans and braided streams form as a result of this slowdown of the river. While it can be difficult to see today with the level of urbanization in Los Angeles, many of the “foothill” communities – Pasadena, Sierra Madre, Tujunga, etc. – were all built in part or entirely on the alluvial fans of the San Gabriel Mountains.

After exiting the foothills, the river transitions to flatter ground. During its journey out of the mountains the river has had other tributaries join it. The river now has a much larger channel and discharge. However, since it is now traveling through terrain that is gently sloping towards the ocean, the river is now moving much slower. One method a river uses to slow itself down are meanders – curves in the river. The more curves there are the slower the water will flow. The now lower velocity means a lower energy level, and thus a decrease in competence. Now, only small grain sizes can be moved (clays and silts) downstream and the riverbeds typically contain sands and gravels.

Before, in the mountains, erosion was vertical – down cutting. Now, on the flat river plains, erosion is lateral – side-to-side. This forms a “U” shaped valley with features such as floodplains, meanders, yazoo tributaries, backswamps, and oxbow lakes. These last two features – backswamps and oxbow lakes – are types of freshwater wetlands.

Before the river enters into the ocean it will flood the low lying coastal areas with water. Several types of coastal wetlands can form as a result - estuarine, freshwater, and seasonally flooded.

Finally, after many miles, the river enters into the ocean. The ocean is basically standing still compared to the river. The river slowly decreases its velocity, which then decreases its competence, and nearly all of the sediments carried in the river are quickly deposited. Two types of aquatic environments will form as a result: coastal wetlands and deltas. Coastal wetland areas are an important part of the river as they help slow down the water flow. This allows for the deposition of sediments before reaching the ocean. Any sediments not deposited in the coastal wetland areas will form a delta. The size and shape of the delta depends on a lot of factors, included the amount of sediment that is being deposited, the depth of the continental shelf, and whether the coastline is a submergent (lowering into the ocean) or emergent (rising up from the ocean).

The trip:

The Arroyo Seco has always been a beautiful place. The Spanish named it “Arroyo Seco”, or “dry creek”, as the river only has water in it seasonally. However, the river we call today the Arroyo Seco was called Hahamongna by the Tongva – “place of flowing waters and fruitful valley”. Alternatively, it could have also been named for a Tongva village that was located near what it now JPL. Unfortunately, the origin of the name has been confused over time. President Teddy Roosevelt was so impressed by the beauty of the Arroyo that he wanted it to be persevered as a National Park. It’s too bad that never came to pass!

The headwaters of the Arroyo Seco are high in the San Gabriel Mountains, and the river extends south through the cities of Pasadena, South Pasadena, Highland Park and the Montecito Heights and Lincoln Heights neighborhoods of Los Angeles before merging with the Los Angeles River. This trip will follow the Arroyo from start to end so you can see not only how the river changes as it flows from the mountain to the valley, but also how humans have try to “manage” the river for our own needs.