Weathering, Erosion, and Deposition: Rivers
How does the Earth change over time? How long would it take my yard or neighborhood to become unrecognizably different? The answers to these questions depend upon your climate, the shape of the land, where you are on Earth, and even how humans behave! In this book you will learn about how natural forces, especially the force of water, can build up and wear down the land.
THE WATER CYCLE
The water cycle, or hydrologic cycle as geologists call it, is the never-ending transformation of water from solid to liquid to gas, and its continual travels from place to place. Two parts of the water cycle are commonly misunderstood. Evaporation means the change of any liquid to a gas. In the water cycle his happens not only to the ocean but to any other exposed water, even beads of sweat on skin! The sun provides the energy for this part of the cycle. Evaporated water is invisible and is called water vapor. Condensation is another difficult term. Condensation means the change of a gas to a liquid. When water vapor changes to liquid, it is condensing. The tiny droplets that form from condensation make up clouds. They are so small that they can be moved by air, but they are big enough to see. If they grow large enough in the cloud, it will rain, sleet or snow.
After precipitation reaches the ground, it may evaporate again or go through other parts of the cycle, such as becoming groundwater, entering the ocean, or going through a tree or other organism. The part of the water cycle where water runs across the ground is when water may have a huge effect on Earth's appearance and people.
What happens in your neighborhood when it rains? A little rain can interfere with our plans, but it can also be refreshing. If you have ever experienced a drought, you can appreciate that rain is necessary for plants, animals, and human life. As it is raining, several things are happening at once. Some water re-evaporates before it hits the ground! Plants and their leaves are slowing down the raindrops. Some water is soaking into the tiny spaces between soil and rock particles. Some water runs on the surface before evaporating or soaking in. This water is called runoff .
Runoff will flow until something changes. Some runoff finds a level area and makes a puddle. Some eventually soaks in or evaporates. Other times, runoff joins a river that flows all the time. (Geologists call all permanent flowing water a stream, whether it is a brook or a big river.) Runoff is an important contributor to the water in many streams, but it is not the only one.
WHERE DO STREAMS GET THEIR WATER?
Runoff, as we have seen, can end up in a permament stream. There are other places a stream can get water, too. The beginning of a stream is called its source or headwaters. Sometimes this could be a single spot, but other times there can be many spots close together that add to the water. Water can come from springs, where groundwater comes to the surface; from lakes or ponds; or from melting snow and ice. Most large rivers also have tributaries , which are other, smaller streams that contribute or give water to the main river. A river with all of its tributaries is called a river system.
The geographic source of a river-- what you would see marked on a map as the begining-- is a small area, but not all the water reaching the mouth or end of the river is from that one spot. To truly understand and protect the water moving in a river, we need to consider its entire watershed . A watershed is all the land surrounding a river system, or all the land area whose runoff ultimately drains into a certain river.
Think back to that rainy day mentioned earlier. What if some of that rain doesn't soak in but runs off the land into a small brook? From there it may join bigger and bigger rivers, for example the Merrimack River in New England. In that case, that land is part of the Merrimack's watershed. The Mississippi River in the central US has a huge watershed; the land area that gives runoff to it comprises about 40% of the continental US! Most watersheds drain ultimately to the sea.
What makes runoff, and streams, flow in one direction and be part of one watershed and not another? Another word for watershed is drainage basin, and it can be helpful to think of a sink basin. The edges are high and the water flows down towards the drain unless you splash it past the edges, in which case it will run somewhere else. The outline of a watershed is high ground, and since gravity always requires water to flow down, any runoff within that outline will flow down to the main river. The high ground that surrounds a watershed is called a divide.
What goes on within a watershed can affect other areas of that watershed. If a dam is built, if water is removed from a stream, if a storm comes, or if people make other changes to the land surface, they can affect the flow and quality of the water. In future readings we will focus on the effects pollutants can have on streams.
WHAT ARE WEATHERING, EROSION, AND DEPOSITION?
When you walk around outside, if you aren't on a human-made surface, you are on some sort of rock material. Plants such as grass grow in soil, which includes lots of tiny bits of rock. The beaches you visit have sand, which is made mostly of tiny grains of rock. You could just be standing on a rock! Where do those bits and pieces of rock, called sediments , come from?
Vast bodies of rock have been and are still being formed by the processes of the rock cycle. After forming, they may be exposed to water, ice, plant roots, and other rocks. These will ultimately break rock into smaller and smaller pieces. This is called weathering . Chemical weathering involves chemical changes, such as when acids change rock into new substances. Mechanical weathering involves roots or ice physically splitting rock by getting into cracks or similar processes.
Once rock is weathered or broken into smaller sediments, it may then be moved. The process by which ice, wind, water or gravity move sediments around is called erosion . Erosion transports sediments, sometimes very far from where it was formed. Erosion is considered a destructive process because it takes sediments away. That word doesn't mean erosion is necessarily a bad thing, it just means it tends to reduce elevation and wear away land.
At some point, for various reasons, water, ice, wind, or gravity will drop sediments in a new place. This process is called deposition . Deposition is considered a constructive process, because where sediments are added, the land is raised or built up.
WHAT FACTORS AFFECT EROSION AND DEPOSITION BY SURFACE WATER?
When surface water (runoff and streams) flows, it might carry or erode lots of sediments, or very few, or it might do deposition. It will depend on many different factors such as the slope of the ground, the type of ground surface, or the weather.
Gravity is a constant, important factor, because it pulls all things close to Earth's surface towards the center of the Earth. Because gravity makes water always flow down, the steeper the slope, the more potential energy water flowing down it has. This potential energy could turn to kinetic energy, making the water flow faster. But a stream can also use its energy eroding its bed, or stream bottom. So, a steep stream isn't always fast. A stream that is not very steep, though, can't ever get very fast or erode its bed much.
For runoff, which flows only after rains, the type of surface it flows on is a very important factor for erosion. Plants prevent erosion in several ways. They slow down raindrops with their leaves. They are always taking water from the soil for use in photosynthesis and transpiring it through their leaves, which removes water from the soil, making it less waterlogged. Finally, plant roots have tiny root hairs which grip the soil and keep it in place. The more healthy vegetation in a place, the less sediment erodes.
Wherever there is pavement, runoff cannot soak in during and after rains. Water "piles up" and tends to rush down streets, picking up speed and erosive power. When that water finally reaches an area without pavement, it has more power to lift sediments that may be there. Such human-assisted runoff during big storms can wash out roads and damage the landscape.
SPEED AND SEDIMENT
If water, either as runoff or a stream, speeds up, it has more energy and becomes moreable to lift heavier and heavier sediments. This can happen during storms, for example.
When water slows down, it will have less kinetic energy , and is less able to erode sediment. This typically happens when the water reaches flatter ground. It cannot hold as much sediment as when it is moving quickly. Therefore it will begin to deposit sediments. It will always drop the heavier sediments like pebbles first, then- if it continues to lose energy- it will drop sand, then lastly it will drop fine material called silt. The way that slowing water always drops bigger sediments first, then smaller and smaller ones later is called sorting.
Geologists can take advantage of sorting to figure out what happened in the past. They can look at sediments or rocks made from sediments, and they can tell what kind and direction of flow once deposited them by how they are sorted.
THE IMPORTANCE OF SORTING
Sometimes mixed-up deposits are found. If sediments are all jumbled regardless of size, we call them poorly sorted. They may be a sign of past flooding or may mark the edge lines of ancient glaciers. Flood deposits and some deposits by glaciers are poorly sorted because they happen too suddenly- the water had no chance to separate sediments by size. It is important to study flood deposits because many rivers flood at somewhat regular intervals. Understanding past floods can help us cope with future ones, since floods can cause erosion and deposition similar to many years worth of everyday changes!
Streams Carry Sediment in Three Ways
Geologists consider three ways that a stream can erode sediment: bed load ,suspension load , and solution load .
Whatever bits the stream rolls along its bed is called the bed load . Sediments that are just a bit too heavy for that water to lift will roll, hop, and slide along the bottom. As they jostle and bump, they may experience mechanical weathering, too. We can't easily see the bed load being eroded.
Whatever sediments are we see float along throughout the water are called the suspended load . This load is easily visible, as it makes the water look cloudy. Often it is the fine silt particles that are able to float, but a fast moving stream could carry sand or even bigger particles in suspension. The suspended load has big effects on life forms, as we will learn later.
Finally, rivers are able to dissolve a lot of materials. These materials include things like minerals and some pollutants. This solution load is invisible. Some minerals such as calcite are easy for rivers to dissolve. A large, long river erode more weight in the form of dissolved materials than it does as a suspended load!
STREAM FEATURES
What are the results of all of this sediment being eroded and deposited? We see them in many features of the landscapes around us.
Steep streams, sometimes called "young" streams, have lots of potential energy. They often use lots of that energy making whitewater, burbling loudly, and eroding their beds. If you have hiked in mountainous areas, you know that sometimes you hear such streams before you see them. They essentially dig their own streambed deeper and deeper, creating a V-shaped valley with themselves at the bottom of the V. The beds can be rocky, because the stream long ago took all the lighter sediments away. Where a steep stream cannot cut its bed because the rock is too tough, we will have waterfalls or rapids.
Less steep streams might not have as much energy, but they probably have more water because their tributaries have brought it. A less steep stream can be called "mature." Small variations in the depth of the stream can lead to different areas moving at different speeds. Faster areas do more erosion, eating away at the bank there. Slower areas do some deposition, building up the banks by the slow areas. This forms a curve called a meander .
A meander tends to grow more extreme over time as more is eroded on the outside of the curve by the faster current, and more is deposited on the inside edge by the slower current there. In a flood, the water may cut off the curve completely, turning it into an ox-bow lake that is not attached to the river any more.
MORE SURFACE WATER-MADE FEATURES
Another feature that forms due to floods around "mature" and "old" flat streams is a floodplain. When a river floods its banks, the floodwaters encounter vegetation and other obstacles. They slow down as they spread out beyond the banks. As that water slowly soaks in and/or evaporates, it drops lighter, silty sediments everywhere. Over the course of many flood events, the floodplain is built up. This silt makes excellent, fertile soil for farmers, but it can be dangerous to live there. The next time the river comes over its banks, it will spread out across the flat floodplain and add to it.
"Old" is the term used for the most flat rivers. Typically a river is flatter where it approaches the ocean. It may have a wide floodplain, many meanders and ox-bow lakes. Over time it wanders across a broad valley. Most likely the river will end at a lake or the ocean. This end of the river is its mouth .
An ocean (or lake) is level, so when the river enters there is no way to go further downhill, and water must slow to a stop. The water has no more energy and it has to drop its sediments in the mouth. These sediments pile up and become a delta . The Nile River delta is famous for its fertile soil and its triangular shape, for which all deltas are named, after the Greek letter delta (Δ).
Some rivers end in the sea or a lake but do not form a delta. Such a river may not have carried many sediments to begin with. Ocean currents or waves may have moved the sediments away, or deposited them to form a beach . In fact, the ocean moves sediments around shores all the time. Wind far out to sea creates the waves, and when waves approach a beach, they tend to come in at the same general angle. However, the wave water sloshes out directly downward. Little by little, this combination of movements pushes the sand parallel to the shoreline. This is called a longshore current .
