5.4: From the Mountains to the Sea

Our search for the origins of beach sand begins in the most unlikely of places, the mountains. This is where many sediments begin their life. Geologists refer to the source material that produces sediments as the parent rock. The rock(s) from which sediments originate defines the sediments’ heritage—the properties inherited by the sediments from the parent rock. Just as your heritage may be defined by the genes and cultures of your parents, the heritage of a particular type of sediment comes from the characteristics of the rock that gave rise to it. Sediments have parents, and they inherit properties from those parents, just like us.

To understand how sediments move from the mountains to a beach, it’s useful to introduce the concept of a watershed, an interconnected region of waterways that drains water, dissolved materials, sediments, and debris to a common outlet. Watersheds drain water from higher elevations to lower elevations. Eventually, the water and any associated dissolved or solid materials come to rest in a lake, a manmade reservoir, or the ocean (e.g., Dobson and Beck 2022).

Most watersheds are associated with rivers, and these may be supplemented by smaller creeks and streams, what are known as tributaries. A river’s watershed includes all the tributaries that feed water into the main artery of a river. In outline, a watershed may resemble a branched tree, where the smaller branches (i.e., creeks) merge into larger branches (i.e., the streams) and the larger branches connect to the main trunk (the main channel of the river). You might think of these pathways as being like capillaries that connect with veins that merge in the heart. All paths lead to the same place. The waters where the river begins are referred to as headwaters. At the other end, the common outlet of the watershed is called the mouth (e.g., Dobson and Beck 2022).

As one example, the Santa Ana River Watershed—the largest watershed in Orange County, California—represents all the creeks, streams, and rivers that flow into the main body of the Santa Ana River, an area of 2,840 square miles (7,356 km²; SAWPA 2019). The headwaters of the Santa Ana waters lie near Mt. San Gorgonio, the highest peak in Southern California (the seventh highest in the lower 48 states). The mouth of the river—some 100 miles (161 km) downstream—opens between Newport Beach and Huntington Beach and empties into the Pacific Ocean.

The concept of a watershed is an important one for understanding how each of us plays an important role in protecting the ocean. Anything thrown or poured on the ground—even dozens of miles from the shore—represents a potential pollutant. With sufficient flows—which occur during heavy rainfall or rapid snowmelt—this material enters the watershed and flows to the ocean. As Long Beach oceanographer and Algalita founder Captain Charles Moore says, “The ocean is downhill from everywhere.”

Weathering

We now return to the three most common minerals found in Southern California beaches—quartz, feldspar, and biotite. Based on what we just learned, we know that they must have come from a rock containing these minerals. As it turns out, we find these minerals in granite, a common igneous rock. Could granite from the mountains be the parent of beaches?

Next time you venture into your local mountains, look carefully at the foot of a rocky outcrop. You will see chunks of smaller rocks and sediments broken off from the main rock. These sediments formed as a result of weathering, the disintegration and alteration of rock at Earth’s surface. Geologists recognize three major types of weathering (e.g., Carroll 1970):

• Physical (or mechanical) weathering, the breakdown of rocks by physical processes
• Chemical weathering, the breakdown of rocks by chemical processes
• Biological weathering, the breakdown of rocks by biological processes (some geologists lump this category with the other two because organisms can physically or chemically break down rocks)

Physical weathering refers to any process that bangs rocks together and breaks them apart or physically separates them through wrenching or brute force. Rockslides, the tumbling of rocks downhill, and earthquakes, the violent shaking of the ground (which also causes rockslides), are two examples. Heating and cooling of rocks—thermal expansion and thermal contraction, respectively, that wrench and weaken rocks—is another. Moving water—rivers, waves, currents—and even strong winds can clang rocks against each other and cause them to fragment. And water that freezes and expands in the cracks of rocks causes a type of weathering called frost wedging.

Chemical weathering occurs as a result of dissolution (i.e., dissolving) of the more soluble parts of rocks, usually by water. Some rocks contain salts, which, of course, readily dissolve in water. In fact, dissolution of salts in rocks and drainage of the saltwater into a basin create hypersaline lakes. Rainfall, too, plays a role. Raindrops naturally dissolve carbon dioxide in the atmosphere. By the time they reach the ground, they are slightly acidic. The slightly acidic rainfall is responsible for the weathering of statues and tombstones, which makes them appear as if they have melted.

Biological weathering refers to the activities of organisms that contribute to the fragmentation of rocks. Lichens—a fascinating partnership between fungi and algae—secrete chemicals that accelerate rock decomposition. Trees are very good at wedging their roots between cracks and physically breaking rocks apart. Think about that next time you are skating down a sidewalk and come to an abrupt “ramp” caused by a tree root.

All of these weathering processes can produce prodigious amounts of sediment. Consider that more than 30,000 feet of sediments (more than 9 km)—nearly as deep as the Mariana Trench—fill the Los Angeles Basin at its deepest spot beneath Downey (Yerkes et al. 1965). That sediment came from the surrounding mountains. But how did it get there?

Sediment Transport

Scientists who study sediments—sedimentologists—refer to the movements of sediments over space and time as sediment transport. For the most part, sediment transport refers to motions of grains due to moving fluids, such as winds or moving water (e.g., rivers, currents, street runoff). But gravity-driven flows, such as those that occur when glaciers push sediments downhill, also contribute to the movements of sediments.

When a moving fluid encounters a particle, it exerts a force on the particle. The force exerted by the fluid depends on a number of factors, such as:

• the properties of the fluid (i.e., its density and viscosity)
• the speed of the fluid and the nature of its flow (i.e., steady or turbulent)
• the size, shape, density, saturation, and compaction of the particles
• the shape of the surface on which the particle rests
• other factors

Now, like us, a particle can withstand a certain amount of flow without budging: we don’t get blown away every time the wind blows. However, there comes a point when the fluid motion is strong enough that the particle begins to rock and roll. The fluid speed at which a particle begins to move is called the threshold velocity.

Three definitions describe the motion of an individual particle in a moving fluid. The particle can tumble across a surface, generally (but not always) in the direction of fluid flow, a process called rolling. The particle can hop, that is, it can become temporarily suspended in the fluid and move forward before landing back on a surface, a process called saltation. Or the particle can “fly” within the fluid: it can move as part of the fluid, a process called suspension. In most environments, an individual particle will experience all three of these processes—rolling, saltation, and suspension—as the motions of a fluid generally vary with time and distance. (See Biju-Duval 2002 for a more extensive treatment of this topic.)

Sediments in Motion

Now, let’s stop and think about these simple concepts as they might apply to sediments, and especially to different size classes of sediments. Fluids such as water or air exert a force on objects immersed within them. Fluid velocity determines whether a given particle will roll, hop, or become suspended in the flow. The faster the fluid, the more likely a given particle will become suspended.

However, the likelihood of a particle taking off, so to speak, also depends on the size of the particle. Smaller particles move more easily than larger particles. Consider your own experience with house dust, which consists of particles in the silt and clay size fractions. A swift blow across a dusty surface quickly sends the dust into the air (i.e., the dust becomes suspended). Yet try this with a boulder, and you will likely become winded before the rock budges. Smaller particles—ones without a lot of mass to resist fluid motions—will move sooner and become suspended faster than larger particles. Put another way, we can say that smaller particles have lower threshold velocities than larger particles do.

Given this, it should make sense to you that the threshold velocity of particles increases as the size of the particles increases. Specifically, clay-sized particles have lower threshold velocities than silt, sand, gravel, and boulder-sized particles, which have higher threshold velocities. From this, it should also make sense that clay-sized particles will move under a wide range of fluid velocities while boulder-sized particles will only move at the highest fluid velocities, a narrow range of fluid velocities.

Of course, the type of fluid will make a difference, too. Eight hundred times denser than air, water packs a punch on anything in its path. If you’ve ever seen video of fast-moving water from a torrential rain, an overflowing river, or a tsunami, you’ve probably seen cars floating downstream. In places where flash floods occur, you should be familiar with the National Weather Service warning “turn around, don’t drown.” It takes only six inches of water to knock a person off of their feet and only a foot of water to levitate a small car. So while strong winds can make suspended sand feel like sandpaper on your bare legs, the wind, even at hurricane velocities, is not likely to move cars. But water moving at even moderate speeds can move boulders and larger objects.

If smaller particles move more easily, then it stands to reason that they also move more often and over greater distances than larger particles. Particles of clay, silt, and even sand will be carried by wind or moving water much more frequently and much farther than gravel and boulders. Trade winds can blow clay-sized particles clear across the Atlantic Ocean, from the Sahara Desert to Florida and Texas (e.g., Toon 2003; Goudie and Middleton 2006; Conway and John 2014). Particles of ash from volcanoes or large fires may be carried around the world (Carn et al. 2015; Peterson et al. 2018). So, too, water moves clay- and si (97)00066-1" onclick="javascript:window.open('https://doi.org/10.1016/S0146-6380(97)00066-1', '_blank', 'noopener'); return false;" rel="noopener">Hedges et al. 1997).

Sorting of Sediments

As a result of transport by fluids, different size classes of sediments separate from each other. Larger particles stay close to their source while smaller particles move farther from their source. The size separation of sediments that results from their transport by moving fluids is called sorting. If you’ve ever sorted laundry—separating socks and undergarments from shirts and pants—you have some idea of what sorting means. Generally, high winds and strong currents sort sediments faster than light winds and weak currents. Alternatively, light winds and weak currents acting over long periods of time will also sort sediments. Your mom may sort laundry faster than you, but given time, your slow and steady pace achieves the same result.

Because winds and currents sort sediments, sedimentologists can learn a great deal by studying the degree of sorting in a given deposit of sediments, a characteristic called the grain size distribution. If the range of grain sizes in a deposit is narrow, that is, if the grain sizes are similar, the deposit is said to be well sorted. Alternatively, if a wide range of grain sizes is found in a deposit, we can say that the deposit is poorly sorted (e.g., Folk and Ward 1957). Can you think of why a sample might be well-sorted or poorly sorted? (Hint: It has to do with fluid motions.)

Modification of Sediment Grains

All this moving causes sediment grains to lose some of their original character. Sedimentologists use the term texture to refer to the size, shape, and arrangement of particles in sediments or sedimentary rocks. As particles slide, hop, and glide in a moving fluid, they jostle and bump into each other, a kind of sedimentary mosh pit. The collisions between particles slowly wear them down and smooth out their rough edges. Particles that were initially angular in shape take on a more rounded appearance. If you’ve ever walked in a dry riverbed or along the shore of a lake with a cobble beach, you may have noticed the flat and smooth rocks. That’s because these rocks have been modified by water as it flows over them and tumbles them against their fellow rocks. The action of the water works a lot like a rock tumbler, a device with a rotating drum used to polish rocks. Thankfully, nature does the same thing. How would we skip rocks across the surface of a lake if they weren’t round and smooth?

Where the River Meets the Sea

Finally, we return to the beach. Let’s review the journey of a grain of sand from the mountains to the beach:

• Weathering and fragmentation of parent rock into sediments
• Erosion and transport of sediments away from their parent rock
• Modification of sediments as they roll, jump, and fly downstream
• Deposition of sediments at a river’s mouth or transport out to sea

Each of these steps unfolds over timescales of days to decades. In Southern California, rain falls mainly in winter, so weathering, transport, and modification of sediments will be greater in winter. In late spring, the Santa Ana River dries up. In fact, most times of the year, all you’ll see is a dry riverbed. Little change in sediments or their properties will occur at these times. But when we get significant rainfall, such as happens in January–March, or during periods of El Niño, the river becomes a torrent. Higher flows mean a greater volume of sediment is transported and that the sediment will travel farther from its source.

Satellite images following extreme rainfall show plumes of sediments being carried offshore (e.g., Nezlin et al. 2008; Holt et al. 2017). These images likely represent suspended silts and clays, the smaller and more easily suspended sediments. Sand-sized sediments tend to be deposited at the mouths of rivers. There the energy of the river diminishes as the river enters the ocean. Slowing below the threshold velocity of suspension, the sand settles onto the river bottom. These deposits of sand at the mouth of a river form deltas—a landform shaped like a paper fan, a bird’s foot, or other form. But deltas don’t always form, and sand doesn’t always pile up at the mouth of a river. Let’s see why.